Forza V would be more advantageous to drop this year, if its one or the other. Next gen sim racer uncontested in europe would be a very strong move for MS. Drive Club is a new IP, so one could assume that its sales results probably won't be seriously challenging to MS until a potential second installment, based off the assumption that a new ip arcade racer will do "okay" at retail, but not megatons of sales, which I think is a fair enough assumption.
Then again, drop both of them, and you could cover both sides of the racing market easily, and likely not really see any sales cannibalisation; I think the crowd getting sim racers would differ from that of the crowd getting PGR5, with minimal overlap. Or, yes, send PGR5 off to be the spring sales chaser. Forza is definitely the stronger candidate to move boxes though, if it does get released before GT7, with more brand awareness, etc etc. Would basically shift units itself. PGR5, not as easily, I don't think.