I have both and love both. If I had to pick one, I would pick the leather-- I think they are more versatile and can be worn easily all year long.
If you haven't ordered yet I would advise you to not order from Gucci.com, but from one of he department stores--
Nordstrom , NM , etc-- if you can. I just don't think Gucci.com has its act together.
Here's my story......... I ordered my first pair of black leather Marmonts from Gucci.com at the beginning of April and was due for delivery in two weeks. I ended up with two big issues :
1) I actually received the shoes mid-August and at no time during the 4 months between April and August did Gucci ever communicate with me-- just the original order confirmation in April. I called them in May, June and July and although customer service was very nice, I got a different story every time I called .
2). I received the shoes in mid-August and was thrilled to get them. I checked the box for the size - 38- but didn't try them on since I already had the suede and knew 38's were good. In mid-September I pulled them out to wear and noticed one shoe felt tighter than the other -- turns out they had sent me one 37 1/2 and one 38.
So 5 months after I ordered from Gucci.com I still didn't have the right shoes. I sent them back to Gucci.com and they refunded the purchase price but I won't order from them again.