Ubuntu? Debian? Something else?

[Changes]

I found an eBay notebook for a good price, so this is going back to the garage.
It's a 700mhz P3 with 256 megs of ram (but I can add 128 more easily since I have a 256 SODIMM module sitting around) and a 20gb hard drive.

Now that I can install a serious distro without worrying about overtaxing the computer, I am again in the situation of not knowing which distro's better.

Requirements are:

1) no dependency hell. This is very important: I remember coming close to ripping my own hair out while trying to install stuff on Mandrake some two years ago. I don't have enough sanity left to go through that again. I want to download a package, tell Linux to install it, and have it seamlessly download and install whatever other packages might be necessary.
2) as user-friendly as possible.
3) kinda light if at all possible; this computer is newer, but it still isn't a modern-day P4 with two gigs of ram.

From past discussions (and lurking) in other boards, I got the feeling the two best distros for my needs are Ubuntu and Debian, but I don't know the differences and which is best for me, or if there's something even better.

A little help would be much appreciated

[devils casper]

Both, Ubuntu and Debian are good but with 256MB RAM, Ubuntu might be a little bit slow.
check this POLL. Fav Distro for Older Hardware.