Campaign Quandary

The C&G gang are planning on starting a 40k campaign which will use Mighty Empires as the background mechanic for our games. My quandary is that I want to use a new army, as I've played a lot of games with my Chaos Marines and, frankly, I feel like a change! My problem is, I'm glacially slow at building new armies and I really don't want to rush my labour of love (ie, the Squat army I bore everyone with).

So, if I'm not to use my Chaos Marines, what am I left with? I can muster about 1,000pts (1,500pts at a stretch) of Tyranids, or I could field a reasonably balanced Eldar army (up to 1,750pts).

Guys - any preferences on what you'd like to play against?

G.