Which do you think is better? And why? Here's my opinion:
I feel that SP is a lot better than MP. For a start, I find multiplayer pretty boring a lot of the time, and SP is more exciting. SP allows you to get immersed in a story line, and have something more interesting to do than blow people's heads off. MP gets incredibly repetitive.
Second, just a point I feel I ought to make: Saying SP is too difficult to map is not really a suitable excuse. SP is the basis of everything, and like Andy said, most MP maps fail because people haven't tried to understand the SP basics, and gone straight to multiplayer. It's all down to practice, and a lot of the tutorials on this site are for single player maps. It's not difficult, and once you have a good grasp of the basics, it's easier to experiment with new entities.
I started with SP and I now find it very hard to map for MP because there is so little you can do with it. SP has so many more things you can do, and look at all those mods and map packs on Ten Four (http://tenfourmaps.telefragged/com). A lot of them are outstanding, and show a lot of very hard work. Just play one, and it'll probably take your mind right off multiplayer.
I feel that good SP mods encourage people to map for SP because you'll probably think "I wish I could do that", and you'll persist until you can. And as soon as you can make ends meet, they move the ends, and you have an even higher standard to work towards.
OK, that's my opinion. What is yours?
Seventh: I was a busybody and stuck paragraphs in here because otherwise most people wouldn't bother reading it. I'll take them out if you want