all the AAMC MCAT guide says is to have a black ballpoint pen... ive never been a big fan of erasable pens... if you touch the ink.. it comes off. smears easily, doesnt erase all the way... i really dont think they are going to be like oh look erasable pen... its so much better than the others!!! give it a T!
just go with what they say in MCAT essentials on AAMC's site... everyone else will be crossing out crap so it doesnt matter either way.... purely your decision and what you are comfortable with.