I find them quite different, and both have plenty of appeal.
The rotor tips and other such issues with the Rotary engine tend to put me off a little, It tends to get brought up in conversations about the RX8, along with oil consumption, poor fuel consumption etc, but it's a nice looking car IMO, modern and has presence, it's also not too shabby on the road, although I've only driven the 190BHP version, and at a slow pace, but it seemed taught and together.
The DC2 is just different, older, VTEC, and has track cult status to me, it's getting on a bit, and finding minters is getting harder, with the looks dating quite quickly now.
I think it would depend on what I'm buying it for, I think I could be happy with both, but probably get the RX8 purely because it's still capable, but much more modern, I'd just have to satisfy myself the running costs don't unbalance things.
[edit] I'll keep out the M3 vs RX8, I know which I'd have..