I have the TK21 U2 and the old M21 w/SST-50 and between the two:
TK21 has:
- tighter/brigher hot spot and slightly larger flood beam that will offer more throw
- easy one-handed mode change with a click of the button and quick access to hidden strobe vs. using two hands and twisting the head while going through strobe to do mode changes
The M21 can accept 2 x RCR123's in case that's all you have (the TK21 can only handle up to 2 x CR123's).
Overall my vote would go to the TK21 but again this is based on a comparo to the older M21. You can check out a review of the TK21 via the link in my sig line and HKJ has posted a review of the newer M21-X
here.
Hope that helps and good luck with your hunt.
Cheers,
Tim
Build quality is about the same although the TK21 feels a bit sturdier.