You can rebuild a MK18 on your own, but MRR( or any IUR) is pretty much out of the question. You will need a very expensive tool ( which is not available) or fork out money to one or two gunshops who have fabricated the tool to swap barrel. The barrel wrench for DD is pretty easy to get and it is cheap.
MRR also does not have QD socket near the barrel nut and the first m-Lok slot is far away from the receiver. Depending on how you employ your sling it could be a problem, as you may run out of space for your support hand with a 11.5" gun. This is a problem with the 9"ish HK slim line hand guard too, as well as BCM KMR, when used with short barrelled rifle. On a MK18, you can attach the QD sling swivel almost right next to the receiver, it gives you more of the valuable "hand" space on the rail. The SAS IUR wisely addressed this issue per SAS's requirement, but not with the civilian MRR.
Other than that, if you don't think you will ever rebuild your guns, either one is fine.