Hi CL,
I think your in an enviable position of even considering the purchase of these two fine watches (3970/5050).
My opinion will appear bias (as I own a 5970), but I will try to give you an objective opinion nonetheless.
3970 vs. 5050:
I Like both watches , however I find the 3970 , due to it's thickness, gives the appearance of being bit small and thus lost on my wrist. Notwithstanding, it's still a beautiful watch, especially in pink gold.
I love the 5050, and it's a much more rare watch then the 3970 (correct me if I'm wrong, but I think the production run , for the 5050, was only approx. 7-8 years, vs. 18 years for the 3970???).
I'm actually also looking for a 5050 to add to my small collection, that's how much I like it.
Having said that, IMHO, I would chose my 5970wg over any of the above watches, ...hands down. I love the dial, with tachymetre, and the perfect size.
My 2cents.
Good luck.