Flightmaster without question. I think it is a more wearable watch, the 125 being a real lump. I also prefer the aesthetics of the Flightmaster, and its functions. Both watches need to be in good condition to look their best and do not take polishing in any shape or form.
Both have great movements, the Calibre 910/1 based on the Speedmaster 861 with added GMT complication, and the 125 has a unique Chronometer rated 1040 auto, which has a great heritage. Manual watches always speak more to me though.
I think both watches are quite plentiful on the market, even though they are relative rarities compared to equivalent Speedmaster Pro models. The 125 is easily built up with service parts and a non chronometer rated 1040 movement, which is something to be careful of, and I would prefer to land a version with the case back mystery serial number engravings of the early run.
Value wise I think there is 拢1k between each models starting with the highest valued 910, then a 911 finishing with the 125