Well its seems that you already understand the dilemma of your situation..... you have laid out the choices as:
a) get the last generation Z77 and keep your CPU and GFX card because the MoBo will be compatible w/ existing parts
..... or
b) get new generation Z87 and get a new CPU along with it. You seemed to imply that the Z87 wouldn't be compatible with Z87 but that is not the case, the 670 will be just fine....
I gotta ask, how did you wind up in a situation where you have a 3770k and no compatible MoBo ? From a budget standpoint, if I had a complete and working systems minus just a mobo, I'd get the Z77 ..... however if this is part of a more extensive upgrade .... new case, PSU, GFX card upgrades, HD, SSD etc, Id get a Z87