Comparing PLC and MECEP is akin to comparing apples to pork chops. PLC's target audience is 'off the street' college freshmen through juniors. You either do one ten week session or two six week sessions at OCS. You can be prior service or not to get into it, you can also get selected while a reservist.
MECEP is (per the MARADMIN) A ENLISTED TO OFFICER COMMISSIONING PROGRAM DESIGNED TO PROVIDE OUTSTANDING ENLISTED MARINES THE OPPORTUNITY TO SERVE AS MARINE CORPS OFFICERS. So, yes, you have to already be a Marine to even be eligible for the program. In the past you had to be at least a Corporal to even be eligible, but that's changed, now you can be any rank and apply. If you're talking to an enlisted recruiter who's telling you about how easy MECEP is, then he's painting you an idealistic picture that you need to research a little more before diving right in. KBay and I have successfully completed the program and there are quite a few others who are in it right now. What KBay said is true. Pay, allowances, promotions, leave, TIG/TIS buildup, all nice. Going to college while on active duty, sweet. I don't know the selection numbers/rates for PLC, for MECEP it's typically ~100 per year, the selection rates change, it seems to go up a little every year.
PLC or MECEP you get commissioned, you can get flight guarantees, you go on and do great things (hopefully) as an officer. The big difference is where you start from.