Eric - check back a few dozen posts, and you'll find more on this subject... and you'll notice that I love my 305. Since I haven't heard all of the sounds on the 30 I can't speak 100% authoritatively, but I read that the 305 has the same sound set as the 505 does, plus a bunch more. I don't do dance / drum & bass music at all - I need completely different sounds - but the 305 certainly has a bunch of the damn dance-sound things, 'cause I'm always steering around them!
Regarding youe question about the onboard sequencer vs. Cakewalk, I think you ought to use both. The seq in the 305 is pattern-based, meaning you set up a sequence of 1 to 32 measures, and program 7 sounds plus rythm sounds into the separate "tracks". You're limited to MIDI channels 1-7 on the instruments, and drums are always on channel 10: no way around it. There's tremendous real-time control over the sounds - from filter sweeps to track muting, volume, pan, effects (dedicated reverb, dedicated delay, and a switchable third effect), groove quantize, swing, shuffle... it's pretty cool, but limited to whatever pattern you're playing, and sometimes a little bit harder to program the drums than it should be. You can change the patterns on the fly, but you've got to wait until the current pattern finished - for example, if you want to cut to Pattern a66 from a65 at measure 16, and Pattern a65 is 32 measures long, you can't. Instead, you've got to program a duplicate of a65 that is only half as long, and use that instead. If you map your songs out ahead of time, it's not a big deal, but it's gotten me into trouble in live performance when the band cut into a chorus 2 bars early. The "event list" (Roland calls it "microscope") is so cryptic and difficult to use, you won't use it.
Now, it might seem like I'm ranking on the 305 seq, but I'm really not. It's a very nice feature, and Roland wrote not one, but TWO very decent manuals on using it. And, if you have the patience to learn all the nuances of using it, you'll find it's incredibly powerful, especially at the synth's price point.
So, what I've taken to doing lately is doing the drum track sequences on my computer (an Amiga running Dr. T's KCS Level II), and synching the 305 to the SMPTE to do the pattern creation. Once I have those done, I import them from the 305 Into KCS, and then use the excellent event list functions in the software to do the reductionist tweaking. In sales speak, the process is called "leveraging your synergies"!
Hope this helps - feel free to gimme an e-mail if you have specific q's on the 305... but like I said, I'm stoopid on the subject of the 30.