Martin Greenfield are tailored but not slim. They are an updated classic style with beautiful handwork and roped shoulders. The amount of handwork is incredible.
Belvest is a slimmer Italian fit. They are also great suits.
Really you should wait for more "hickey" to show up on Gilt, as they are very modern. Also the Hickey-Freeman "C" body. Both show up on Gilt frequently.
But of the two you mentioned, you will be happiest with Belvest, and you'll get a top-quality...