A Fisher Space pen refill is smaller than a pencil, is more durable, lasts longer as pencils actually do run out (the main advantage is that you can see how much longer the pencil will last) and the Space pen refill works better in wet conditions. Just carry a fresh Space Pen Refill. I wrap duct tape around a Fisher Space pen refill to allow easier writing.

The INKA titanium pen is also very good and allows full size performance but it is ridiculously expensive.

For a full sized pen the Dealextreme Tactical pen is also a favorite.

http://www.dealextreme.com/p/cooyoo-tp3-military-survival-defensive-tactical-pen-48875

Having recently written in the rain using fisher space pen refills on rite-in-the-rain paper, I can say the space pen works far better than pencil lead, even high quality drafting pencil lead. I was also a little surprised at how quickly the rite-in-the-rain paper soaked through.

A pencil is a simpler mechanical device than a pen. Also, SEALs use pencils, 'nough said.

Yeah but the astronauts use spacepens.

I beleive astronauts use pens because graphite particles in zero gravity cause problems.

I use a pencil ever since school science classes - a piece of paper with ink on it gets unreadable when soaked - you can dry pencil written notes out pretty well. (There are fancier solutions but if you are a luddite like me....)

never thought id see a pen vs pencil tread, I go with both cuz i just love being on the fence like that =D

Since were on the topic, lets nerd up a little more shall we? Whats everyone's favorite pencil. I'm like the Pentel .9mm mechanical drafting pencil and the Staedtler Mars Lumograph "B" drawing pencil.

Ticonderoga No. 2.
no moving parts, no extra lead / erasers to carry / lose.
oh, but how do they get the lead inside?

very cool! i knew a pencil was a lead sandwich - look at the end of a never-sharpened pencil - but i didn't know how they did it! and who says we're not technologically advanced? that's impressive.