TxBillyed,
I have a 4100 with a 54" MMM and it suits my needs. I slightly considered a rear finish mower, but decided against it due to my yard conditions. Lots of trees and curves which would be slightly harder to manuever the rear finish mower.
Since the rear finish mower was included in the deal, I guess I'm surprised as to why you're even asking about the MMM. You didn't mention if you purchased it new or used. If it's a new purchase, you may still be able to trade the rear finish mower in for a MMM.
The biggest determining factor in my mind is the terrain and obstacles (trees, bushes, gardens, fences, etc.). If you have long straightaways and gentle curves, the finish mower should do you just fine. If it is the opposite, sell/trade in your rear finish mower and get a MMM.
Terry