Would be pleased to AmStaffer, especially as I do occassionally handle a lab in the show ring and have had a few in my day.
First it must be said that every dog within a breed is different and unique and that there are lines within a breed that tend to be one way or another (i.e. field vs. show). That said, Weimaraners and labs are both companion hunting breeds. Labs were bred for retrieving waterfowl, and Weims originally for big game and are now bred and trained as pointing bird dogs. Both breeds should be very loyal and perceptive of their owner. They will both want to be with you, as part of their companion nature, however... Weimaraners are a bit different, and have a different temperament than labs. Labs, are very amiable, easily directed and do not put up much of a fight to get their own way compared to a Weim. Weimaraners were not only bred to be hunting dogs, but also as most German developed dogs, they have a working side to their temperament. Originally in the German Weim Club, in order to be bred, a Weim needed to pass at least a ScH I test. This gives them an edge to their personality that a lab does not have, IMO. They are fiercely loyal to their family as they perceive it, and can be psychotic when not given a family to love and protect. They do not do well out of doors for both mental and physical reasons. Their short, single coat can not withstand very hot or very cold temperatures, where a labs double, thick coat does insulate them. Weims that are kenneled outside often exhibit severe mental anxiety, being aggressive and obsessive/compulsive. Again, this can be said for many undersocialized, chained/kenneled dogs, but in Weims the odds are much greater of problems.
Weims can be very versatile, and are very active. My motto and mantra is that a Weim can do anything that an owner has the time to train them in. They track, hunt, do obedience & rally, agility, SAR, drug and bomb detection, service dogs and I have even seen them trained as working K9s, sled dogs, herders and carters.
Their intelligence is only matched by their strong will. This will is what makes them inappropriate for the first time dog owner. Opposite of a lab, they want their own way at all times, and through bonding and training you can meet in the middle and have the two wills (trainer and dog) meet.
Labs and Weims are about the same size, Weims being a little taller, labs being a little stockier. The average dog weighs about 70 - 90 lbs, the average bitch weighing in at slightly less 50 - 70 lbs.
As far as health problems. Labs suffer from hip and elbow dysplasia, as do Weims. Weims are more susceptible to bloat than a lab, with their deep chests. Thyroid problems are also a growing concern in Weim circles, as the immune system seems to be declining in strength in successive generations.
Finally, Weims are much less popular than labs (for some very good reasons) and Weims were begun in this country with a very small breeding pool as the Germans did not release many dogs before the 50's. Before buying a Weim, it is imperative that you research a good breeder that knows the breed, its health and what family can handle a Weim.