Mega Drive (Genesis)
Both are great systems but it really comes down to which system has the games you're more interested in. Also Genesis games have been ported many times and are on compilations like Sonic's Ultimate Genesis Collection so you may not even need to buy a Genesis if you have a 360/PS3
My dreamcasts have more hours on them than my mega drives.
Dreamcast...wanted it but could never get it (when it first came out)
Both are good systems and you can't really go wrong with either. I'd say get a DC first since systems tend to be a bit more expensive.