Acquisition, Clemence pieces/Birkins can have different size grains. I have 3 Kellys and two have large grain clemence, the other, a size 28 has small even grains. The smaller grain Kelly is more structured and does not slouch as much as the other two bags.
If you love Clemence, this might be something to think about? I don't know how easy it is to find a small grain in Clemence as my Kelly is a Master Craftsman's bag and I always wondered if that had anything to do with the size of the grain. Was the craftsman able to hand select the leather pieces, etc?? I don't know.
But if you want a Birkin that will keep shape over time, I feel you might want to consider a stiffer leather. Certainly Vache Leige would fall into that category. Togo would, although it's not as stiff as VL. Epsom....there are stiffer leathers than Clemence although baby bull is a very tactile wonderful feeling skin.