Ubuntu still has distinct less/more bins. At least mine does, or the 'more' command is sending different arguments to less.
In any case, to see the difference, find a file that has more rows than you can see at one time in your terminal. Type 'cat', then the file name. It will just dump the whole file. Type 'more', then the file name. If on ubuntu, or at least my version (9.10), you'll see the first screen, then '--More--(27%)', which means there's more to the file, and you've seen 27% so far. Press space to see the next page. 'less' allows moving line by line, back and forth, plus searching and a whole bunch of other stuff.
Basically, use less. You'll probably never need more for anything. I've used less on huge files and it seems OK. I don't think it does crazy things like load the whole thing into memory (cough Notepad). Showing line numbers could take a while, though, with huge files.