I tried them both over the last two weeks. Here are some of the things that I noticed. I primarily run Manilla:

NFSFAN (V1.00):
- Exchange email didn't work for me out of the box, requires a patch
- Leaks memory. I find I need to soft reset my phone every day or two depending on my usage. Memory starts out low, around 50% and slowly gets up to the upper 80's, even with no apps running.
- The latest version seems to have some trouble with internet sharing over Bluetooth. That may or may not be an issue for you.

VIN (23081):
- Lock screen didn't work for me and actually seemed to hang my phone.
- Requires you to find and install MS Office and Live on your own (not a huge deal)
- Has newer version of EzInput keyboard (way better IMO)
- Manilla skin is not as nice as the default one that NFSFAN uses, also has some custom icons. This is purely my opinion, yours may vary.
- Doesn't have a task switcher integrated. This is as much the fault of MS as vin, but it make 6.5.1 unusable for me.