They're both going to perform much the same, neither comes with a battery (*angry muttering about when these were <£60 with a battery*), and both will have that chonky-boi AEP grip.
So, I'd agree on following the heart rather than the head on this one. If you do want to crunch numbers, then magazine price and availability can be a factor, and space for a larger battery is always welcome. Barrel length isn't an issue, as you're working with such a tiny cylinder volume - the CM.121 Deagle actually claims lower power than most other Cyma AEPs.
The mosfet is... well, better than not having one, but my CM.123 runs fine without one on a 7.4V , and with the stock wiring replaced with thicker stuff. What made the big difference was running a high amperage lipo (remembering that amperage is capacity x C factor, so biggerer is generally betterer).
