What platform will be sticking around longer 1156 or AM3?
The most powerful processor that will ever be released on 1156 is the i7 880.
For AM3, it is the Phenom IIx6 1090t or for quad lovers, phenom IIx4 970 (3.5 GHz)
No more CPUs will be released for either socket.
Motherboards with the AM3 socket are not limited to 8x PCI-e mode in a crossfire configuration, if the specific motherboard supports it. No 1156 motherboard will support 16x dual mode.
Both sandy bridge and bulldozer will be released on new sockets. current AM3 CPUs will work in the new AM3+ motherboards but bulldozers will not work in AM3 motherboard.
So basically, they are the same. The 1156 socket has more powerful CPUs but they won't be worth the upgrade when you are looking to upgrade. And both sockets are equally out the window.
i5 is more powerful than the phenom II. Might be worth the upgrade, might not.