Marty I advise you not to go for the Daemons unless you plan on spending a hundred or so bucks for some Plague Bearers of Nurgle. Really daemons is a tough army to play right now and I doubt they will get any advantage from the New 40k rulebook to come out in the Summer. I have played three games with my Daemons and the Chaos Gods did not favor me in the least. Two of the games were against a Blood Angles player, this guy was playing an army he was going to use in a tournament, so I was at a disadvantage to begin with. The first game he wasn't familiar with Daemon tactics and game play so all my army deepstriking infront of him threw him for a whirl and even though he got the charge for the most part, Daemons are nasty in assault. I managed to kill his tanks with my Daemon Prince and took his storm Raven out with my Flamers of Tzeentch. I managed to make the game a tie for kill points, he had about one HQ mephiston I think and some kind of squad left. I had about a few daemonettes of Slanesh and the Masgue left by end game. It was just death all around the table, Khorne would have been pleased. The next game I played with him, He held all of his army in reserves which took away much of what I had helping me in the first game. I hardly took out his guys, and he managed to table me. It hurt a lot for it was the first time I have ever been tabled. It hurt twice as much because it was Blood Angels. I also played a Gaurdsman player and oh I did kill the gaurdsman easy enough, but three Lemons on the table plus multiple chimeras really hurt, I had nothing to take out that many vehicles. He as well tabled me. I really wont play my daemons until I have multiple Soul Grinders and a whole army of Nurgle. At least that way I can stay on the board for the whole game.
If I had to choose, I think Tyranids are a nice choice, painting and game play wise. Though the Eldar are pretty nice, I haven't seen them impress me against my Space Marines, but they do have some nice looking models. Granted though if you just want to play a fun army then Daemons, if you get the right units they can be interesting and an extreme challenge. As well painting the Greater Daemons looks to be a fun challenge, as well as painting anything Nurgle.