Well, on my e-kit I use the TD-10/TDW-1 for most of the main sounds, and a DM5 for a few extra cymbal sounds and odd percussion stuff. There's no reason whatsoever that you can't use either the Roland or Alesis drum modules to trigger a sampler via MIDI to get different sounds. I'm sure that there are a lot of folks who do just that. However, most people are usually so broke after buying the drum modules that they either never take the next step, or wait a long time indeed... After all, you need the e-drum brain to interpret the pad trigger outputs, and once you have that, the built-in sounds really come for free...
I use, and am pretty well satisfied with, the sounds in the modules, with a pretty clear preference for the Roland sounds. This is especially true since I got SoundDiver: it has made editing and tweeking the sounds in
the TD10 much more straightforward. They are much more usable after fine tweeking than as they are out of the box. Additionally, the TDW-1 expander is absolutely worth its weight in gold for helping fine-tune the triggering setup in the TD10 (I use Hart and Pintech pads), as well as making some much improved sounds available.
I'd say to get whatever module you favor, and then really dig into setting up the triggering and sounds to work for your style (hint: this will take time, and can't be rushed). Only then, if you really can't get what you need from it, should you invest the cash and try backing it up with a sampler. The current run of sounds from these products is remarkably usable.
I've lately started tracking to MIDI, and not actually commiting the sounds to an audio track until I'm well and truly happy with them. It's really quite astonishing to me (as a MIDI neophyte) to be able to go back and replace the entire kit with another one, or tweek the snare head up a tad bit tighter and boost the rim sound a few dB, or move the "room ambience mike" a few feet on the fly, on a finished take... It's kind of addictive!