Stay with your supporting dealer.
The 18 inch bar with a .325 chain is standard out here for the MS250, and we sell boat loads of them. It's more to do with what size you mostly cut... the difference in a 16 or 18 inch bars in 12 inch wood is irrelevant. If it's is mostly 18, you'd definitely want a bigger saw, but if only occassionaly it's fine.
As for the 250: Good saw, but the original question was 270 or 250, and the 270 is better saw, and the 280 better still, and the 260 even better, and the 361 still better... and the ... and the... It never ends, so buy the saw you are most comfortable with from the dealer that can support you.