Would you consider a damier ebony speedy? Comparing coated canvas to leather is kind of like comparing apples to oranges. I love Bbags but they show wear very quickly and if this is your first high end bag you might end up disappointed by the quality. If you go for a Bbag consider buying a used bag from 07 or before when they were made from chevre leather they are much more durable than the current bags which are made of lambskin. I feel like LV's coated canvas is a lot more durable. I have a azur galleria which looks great a few years later their is slight color transfer on the back which is kind of to be expected since it's white and the leather edging and shoulder strap look a bit dirty and are starting to patina. So those are things to think about. Both bags will sag with use IMO it's a part of their charm the Bal will sag quicker than the LV.
It depends on your age and personal style.
I have a hard time imagining a 50+ woman wearing Balenciaga (no offense to Bal lovers, but I think it suits younger women better due to the bags' rock & roll style - there are many exceptions of course).
The LV Speedy is a classic piece, though I'm not a fan of their leather (vachette I think) getting darker and darker through the years.
Balenciaga anytime! Unlike the previous poster I think you are only limited by your own mind as far as age goes. I'm 45, a suburban mom to 10-year old twin boys who drives a - shock! - minivan and I still rock my Balenciaga Motorcycle bag that I purchased in the fall of '01 for my 35 birthday. It puts a spring in my step every time I use it!