Sheerin,
I've got say, MARS is a huge challenge - a third of our class got cut on BOTC (that's a lot by Venture's standards...apparently they went 'St Jean style on us'.), and while I only did MARS II this summer, watching the MARS III and IV classes... ho boy. Only 2 of 7 passed MARS IV and got their ticket, and on the MARS III course going on right now, 5 of 20 have already failed - and they haven't even gotten to the hard part yet! Be prepared to be up 'til 0200 planning passages and getting up at 0600 on MARS III to present them and be blasted on all your mistakes... (second hand info from friends who completed MARS III this summer).
That said, MARS training was the time of my life. I can't wait for MARS III. Nothing is so fun or thrilling as conning a ship and learning how to be Officer of the Watch. It's a great job.
Also, thought about the Reserves? I'm currently in fifth year university, and joined the Reserves because while I wanted to join the Navy, I wasn't up for the 9 year deal for Reg Force, especially since I plan on getting a Masters or Law degree. (or both? we'll see.) There are too many things I want to do with my life - much like you, it seems.