Here's a couple points to take into account.
#1 if you go the OCS route, you WONT get any tuition reimbursement - at least that's how it is for Army OCS. I presume USMC is the same way.
#2 I thought the GI bill can only be used for *future* schooling, and it only goes into effect after you served your 3 years or whatever.
#3 Enlisting does offer tuition assistance.
#4 You can do ROTC as long as you have 2 years remaining. Want to start working on a masters?
