I am torn between the two usually. If it's between a pen and a MECHANICAL pencil, give me the fricking pen. Mechanical pencils are so much more trouble than they are worth. Tiny lead that lasts like 2 seconds, Tiny eraser that also lasts 2 seconds, (and if you take out the eraser and lose it, all your lead will disappear quickly). Also sometimes mechanical pencils get screwed up with the part that holds in the lead, so when you are writing it gets pushed back into the pencil and it makes me want to rip my hair out. I like that pens make nice, dark writing and don't need to be sharpened though. Erasing is a huge asset for me though because I have sloppy handwriting.
Pen all the way. I have a brushed chrome Bullet Space Pen that I use for 99.9% of my writting. It was a gift for my 18th birthday. It doesn't leak, never dries and never runs out of ink. HUZZAH!
While pens are more professional, I use pencil's for all of my work. Everyone tends to make mistakes, even on paper, so it's nice to be able to erase those mistakes rather than just making a mess.
Pen, because ink is bolder. Sure, I can't easily correct mistakes, but I can't be bothered to keep track of my eraser anyways. (Actually, mine's been missing for a while.) I'll rarely ever use a pen involving calculations of any kind, but most general note-taking, doodling, and writing tend to be in pen.