I don't have experience with any of them, but here's my take (again assuming they all fit perfect, meaning tight but comfortable, no heel lift, no numb toes, etc.)
The Burton Motos are a support level 3 on the burton scale. Looks like they've got speed laces which I think is a plus (I've met one person who prefers old style laces).
The Rampant is a level 4 on the burton scale, so should be stiffer. They also say it's their lightest boot ever...
I think we're talking about minor differences here though. First and foremost FIT is most important, then probably stiffness (an all-mountain rider would probably like a mid range flex), then ease of entry/exit (speed laces DO help when rushing for first tracks). Price also has to be a consideration.
Here's a little story to hopefully help. My girlfriend has been riding about two seasons now. She rented for the first few days and then bought her first setup. She got Burton Sapphire boots, which are an upper-mid range boot, mouldable liner, heaters, etc. etc. and they are very nice boots. She was having troubles with her bindings though, constantly tightening because they always felt loose to her.
Went to a shop to ask about bindings, and they said it was probably her boots. She tried on about 6 different pairs and compared them to her current boots. In the end she ended up buying some cheaper Nitro low-mid range boots. When she originally bought her Burtons she tried them on along with a few other pairs, and thought she had found the right fitting boots. But comparing them to these Nitros in the store, and taking the time with the sales guy to really check the fitment, she got a better fitting boot. Even though the boots are cheaper and not as stiff as the Burtons, her snowboarding is actually better.
I guess what I'm saying is: I know you said they all fit the same, but I can't stress proper fitment enough.