Start with what traits you like/dislike, what you hope to do with a future dog, what you like dog personality wise, size wise... and we can help you go from there
I personally went through the australian sheperd vs. border collie debate myself, and what swayed me toward aussies was (a lot of reading lol) but also, how their owners describe their personalities and what I LIKE to see in dogs.
Things like playful, bouncy, funny, silly.. tended to pop up when I thought of personalities in dogs I like to see, and being traits more common in aussies, that's the breed I went with (there were so many other factors of course lol but that is an example)
Sheltie.. you've got the size thing. Do you like more smaller/medium dogs?
What are you hoping to do with this dog? agility? rally? hiking partner? companion? therapy dog?
Also, someone once told me "Don't focus too hard on breeds.. start with breeders" and doing that it was REALLY GOT ME.
I looked at each breeds club.. and went through their breeders lists one by one (just..this took FOREVER) and one day, BAM I found a breeder with aussies that I LOVED (personality/temperament/health etc.. wise) I knew that more than being the right breed for me, these were the right DOGS for me and what I wanted in a future puppy