Patience!
Please keep in mind that I have to go based off of mostly memory alone as I haven't lived with a Rottie in a number of years now, but I'll do the best I can based on what I remember of their temperaments and not just their individual personalities.
Training wise: I found my Rottweilers a lot easier to train than my Dobermans. The Rottweilers didn't seem to have to over analyze and think things through like the Dobes do. They just did stuff because there was food and they had to do whatever I was asking of them very quickly because they must eat! They were just so much more eager to learn somehow and less distracted I suppose. They could certainly be headstrong as well, but all in all, they just did as they were told in an effort to make us happy a lot of the time. Often praise was more than enough for them and they'd get as excited for it as a human who just won the lottery. Unlike my Dobermans, who as much as they enjoy seeing us happy, they're also like "ok, glad you're happy, now what?". Usually just praise isn't enough for them. It never is for Keira and with Ripley it depends on the day and where we are and what we're doing. With that said, I do find that my Dobermans, eventhough more difficult to teach something to initially sometimes, grasp things more quickly. I also find my Dobermans softer than most Rottweilers. We did have one rescue Rottie girl who would cower if you so much as looked at her a certain way, but they IME is not typical of the breed and that dog also had more than a few screws loose.
Daily life: Both breeds are very attached to their owners and what I would consider to be velcro dogs, but Rottweilers are more shadow like then your typical Doberman. Rottweilers like to be with you and near you, and they like to know where you are, but Dobermans, if they could, would like to find a way to permanently adhere themselves to you. Just knowing where you are is simply not enough. They think they should be touching you at all times. They both can get pushy and rude about attention, so setting boundaries is a good idea for both breeds, otherwise you'll likely wind up with a rather obnoxious dog.
Exercise needs differ slightly in my experience. My Rottweilers got a lot in the way of training and exercise, as do my Dobermans, but the Rottweilers just had a more laid back sense to them. Now that my Dobermans are mature adults they are pretty content to just go with the flow and do what I want, but if they go for a couple of days without much exercise or anything, they do get a little stir crazy. They just seem to crave more. My Rottweilers got a little restless too, but over all they were more content with not doing much of anything some days than the Dobes can be.
Neither breed does spectacularly in the heat, but I remember summer days where my Rottweilers would be just panting all day and too lethargic to really do anything. They'd lay on the cool kitchen floor in front of a fan and just pant the day away. The Dobermans aren't keen to do things when it's super hot either, but they cope better.
Rottweilers are better in the cold than Dobermans. Most Dobes are pretty sure they might melt in the rain, and going out in the cold of winter would be comparable to a human going out without a jacket. My Rottweilers loves to play outside with us in the snow for a long time without ever seeming to get cold. The Dobes can't stand to be out there for very long at all.
Same sex aggression can occur in both breeds. All of my Rottweilers were happy go lucky with everybody and everything, so I have no experience with it in that breed. But I've known a few SSA Dobermans. Thankfully mine are not what I'd consider SSA. This is a trait that is a strong deterrent to me ever getting another Rottweiler or another Doberman again, though. I feel very fortunate that my dogs have all been (so far) friendly around other dogs regardless of sex, but I know that more often than not that's not always the case.
With strangers I find Dobermans a lot more reserved and indifferent. All of the Dobermans I know are people friendly, but they simply do not care about strangers or a lot of the time, even friends or relatives that they know well. My Dobermans would choose to hang out with my over anybody else any day. They're not shy at all, and will ask for touching from anybody, but their own people are their preference. My Rottweilers, while not "Lab like" in that they'g go home with anybody, were more apt to visit excitedly with new people and friends and relatives.
I find my Dobermans to have a lot more guarding/protection instinct than my Rottweilers ever did. A friend's Rottweiler actually let some drunk walk right into their home one night and was thrilled to show the guy around, whereas I think you'd get quite the opposite reaction from any Doberman I know. My friend's Rottweiler's reaction is not what the breed should do, but I think one of my girls would have done the same. The other ones, absolutely not. But all in all I find that you see a lot more of that in Rottweilers than Dobermans.
Rottweilers shed a lot. Dobermans shed more than you'd expect also, but Rottweilers shed like crazy. They have a short double coat where as Dobermans have a smooth single coat (a lot of people don't realize there is a difference in coat).
I find Rottweilers to be a lot more excitable, clownish and in your face than Dobermans are. Dobes, while they can certainly be excitable and have a sense of humour, there is just a different energy to them somehow.
Both breeds stay adolescents for a long time. Especially the boys. I'd say you're not in the clear until a dog of either breed is at least 3 years old.
As to which breed is better for a first time owner, that depends on what you're looking for in a dog. Rottweilers I feel are slightly easier than Dobermans in some ways, but both breeds certainly provide their own set of training challenges and neither breed is for somebody who is not willing to put a lot of time, energy and effort into their dog. Neither breed is an easy breed. I'd recommend spending time with multiple dogs of both breeds and see which you prefer if you're interested in them. I know for me, after owning both, with as much as I did find training the Rottweilers slightly easier, I'd go for more Dobermans over more Rottweilers any day. Doberman energy and I just click better nowadays.
Both breeds are unfortunately plagued with awful breeders, and both breeds as a result have some very unstable, spooky, weird specimens. Ugly too. Both breeds also have their fair share of deadly health concerns as well. So if you're not one to adopt from rescue, be sure you go to a reputable breeder who cares about their dogs temperaments, conformation, and health.
I'm sure I've probably left something important out, and my experiences may differ from other owners of both breeds, but hopefully that was somewhat helpful.