The 14' is faster, period - downwind and flat provided you are comparing apples with apples. A light specialised flatwater 12'6 may be as quick as a heavier surfy downwind 14' design on flat water, but that's about it.
We've literally done hundreds of time trials and downwinds and there is no contest - the 14's are faster. When a guy on a 12'6 is faster than a guy on a 14' it's got everything to do with the paddler (sorry DJ....

)
Personally I hate going slower, paddling as hard as I can, and seeing the guys slipping away because I'm are on a shorter board well knowing that I'd be back on the pace, if I go back to the 14's.
If you are interested in racing then I really battle to make an argument for buying a 12'6 over a 14', saying that you can just decide which class you want to race in.
It doesn't make sense to me. Surely you want to race with everyone in a race, if you split it into to classes then you are effectively only racing with half the field - and you actually never really know how you did because good and bad paddlers are mixed across the classes.
So unless you paddling purely for fitness and improving your own times (which means you are not racing) there is no reason for buying a slower 12'6.
You indicated you want to go and do the races. Racing means competing against other paddlers and for this you must have the fastest board, otherwise its meaningless.