Hi Dave. The A320 production queue is really short at the moment (compared to 737 and past game worlds). It might be a good alternative if you can afford to wait until the end of the decade to replenish your fleets (I am waiting until then to replace my factory new 727-200Advs with them). But that might be a problem if you are running mixed fleets of older, used aircraft.
If you need an immediately available plane then the MD81 is a good choice and I know of a few people using these to replace 737s and 727s. Just remember that although it looks nice on your airline page, there is no advantage running an all McDonnell Douglas airline. But keeping to two fleets is a good thing.
EDIT: Actually could you provide us a link to your airline please?