I'm selling my ps2 at my local retro game store tomorrow and it just so happens they have both a Saturn and a Dreamcast but I can't decide which one to get.
Get both of course Ok, really depends on what type of games you want to play most, as others have pointed. I personally got a Saturn before I got the DC. I really wanted to play Shining force 3 and Dodonpachi.
Voted for Saturn simply because of the AM2 ports, it depends what you're into. You'll end up getting both like I did anyway, c'mon it's SEGA!
Dreamcast is cheaper (though I think it's going up in price), and has a lot cheaper games (not many US games go for more than $50), and new games are being released for it.
If you can get a Japanese Saturn then it's a no brainer. There far more better games on the Saturn that Dreamcast. Both are great but the Saturn was the unsung hero of the day. The DC put up a great fight, had many fantastic original titles but many of them did get ported to other systems.
If you do get a Saturn (and it's American), get an Action Replay (http://www.amazon.com/Action-Replay...09642158&sr=8-1&keywords=saturn+action+replay). You can play most Japanese Saturn games (and the US never really used the cartridge slot on it, so leave it in unless you want to play online), all but a handful will work. Getting some games online from Japan is cheaper than buying the US versions (I got Panzer Dragoon 1 & 2 for about $20 shipped, either game will easily run $30 for a CIB US version).
I'd say get a Dreamcast. Purely because I love much more games on the system. I tried really hard to like the Saturn, but I just didn't like it. It is one of the only 2 consoles I have after selling off my collection. I have a backwards compatible PS3 and a Dreamcast. Both make me happy
Keep the PS2. A lot of great titles were ported to PS2 from both the DC and Saturn. Look up the Sega Ages 2500 series for example.