if you are going to use rcr123s, then i personally feel the FF3 is a better choice with the flupic module. you get regulated-like output on RCR123s and much more flexibility/functionality from the flupic module. But of course you will be limited to the cr123 and cr2 sized battery tubes to my knowledge.
on the other hand, if you want flexibility of battery types, then the jetbeam would be a better choice, with a wide choice of battery types from its AA tube, AA extension tube, and CR123 tube. the MKII now incorporates various brightness levels and a strobe, which really puts it in good competition with the FF3. i don't know if the levels and strobe rates are user settable on the MKII though.
both are well made, but the FF3 being a custom just has that extra touch in precision workmanship. it's machining is superb and it's a joy to operate. but on the other hand, i believe the jetbeam to be more robust, with thicker/better anodizing and a sapphire window and a cheaper price which to me makes it a better 'rough use' light.