I think
@papertiger and
@880 have summed it up well - the GdP will be more hardwearing but may be more prone to yellow over a long time.
It sounds like you are leaning towards the lamb, tho, and I agree it is beautiful and more luxurious looking and feeling! I don't have experience with this particular bag in lambskin, and if you check forums it's all over the map with corner wear (some saying it's terrible and some saying they haven't had much of a problem). I have heard in general YSL lamb tends to wear better than some others, and I had a toy loulou in lamb that wore much better than I expected, even on the corners.
And if you are a bit of a DIYer and not afraid to try rehabbing, I think there are many things you can do to slow wear and/or minimize the look of it once it inevitably happens. For example, on my Balenciaga wallet, I put a thin layer of clear Fiebings Edge-Kote on the edges to give it some extra protection. Then once wear became obvious, I just dabbed with a bit of matching Angus leather paint and it was covered.
I understand when we pay so much for luxury products we want them to last and perform well, but at the end of the day all natural skins will wear over time. But I love being able to fix minor imperfections so that something classic like this will be used for a long time!