I just got a used GTX 660 TI and I am "relatively" satisfied.
I am gaming at 1680x1050 and did many tests, Heaven, 3DMark etc.etc. and find that the limited 192 bit bus DOES NOT, in any way, make a difference as compared to the GTX 670.
I can actually overclock my memory on the 660 TI with +640 in Afterburner which makes the bandwidth only 10% slower as compared to that of the GTX 670. Even with that massive overclock on the memory, there is only a tiny difference as compared to stock which means that the bus is not even remotely saturated.
MY criteria is therefore another than simply performance:
I have a sh!tty case where many of the oversized cards simply don't fit, this is why I choose the Gigabyte WF 2x WF.
BUT...I have to say I don't like the cooling of the Windforce, at least not for my current PC case. This fan does not blow any hot air out, seeing it like that I would have actually preferred a stock blower over that custom cooler of the Gigabyte.
As to actually answer your question whether the GTX 670 is worth the $100 more? IMO, no. (AT least not in my case but then I am overclocking).
Also, based on my bad experiences with that Gigabyte cooler I would actually choose the MSI.