hello, giselle.....strictly personal preference, of course. Of the two, I prefer Clemence for it's softness to the hand. I do not like togo. That said, I own two togo pieces and one is my first Birkin, which I'll always keep.
The second is a Kelly which I'm wanting to replace with Clemence in the same color ( the leather will probably be just a bit deeper/richer in hue ) because I do not like the feel of the togo on my hands when I play with the bag.....meaning, when I work with the belts, straps, I do a lot of touching with my Kellys opening and closing them and so on so I want a softer touch. To me, the togo is a bit scratchy to my hands. ( I might add that I have very sensitive skin )
Many here seem to prefer the togo, tho, as it does tend hold shape better and it's a very good looking leather.
I also feel that it depends on which piece of leather is used for which bag. Not all togo pieces and not all clemence pieces are alike, obviously. The clemence just tends to be more souple and soft and tends to drape more.
I don't feel that of the two leathers you'll notice a tremendous amt of difference in weight, giselle. When you're carrying a B 35 with your things inside, it will be somewhat weighty. That said, I'm told that the 35 is the most popular size Birkin so, most ladies obviously don't seem to mind the weight.
As for losing it's shape, the B 35 tends to do that more readily than the 30 size. If shape is important to you, you might want to go for the togo or another leather. Vache Liegee is a new leather is very popular and somewhat stiffer. It tends to look much the same as togo but holds shape better.