(velocity switched) Sample layering is important for getting realistic drum sounds. It means that when you hit the pad harder, it makes a different (e.g. louder) sound. If you don't have velocity switching and sample layering, then your drum pad is simply a 'switch'. It's like pressing the 'play' button, you always get the same sound whether you lightly tap it, or pound the hell out of it. However, if your goal is only to play backing tracks, then it is irrelevant, as in that situation, you effectively do simply want to 'press play'.
From the SPD-SX page (http://www.roland.com/products/en/SPD-SX/):"2 GB internal memory, enabling approximately 360 minutes of sampling (mono) without requiring external memory....you can house approximately 360 minutes of audio...Nine velocity-sensitive rubber pads, two external dual-trigger inputs"
From the Yamaha DTX-M12 page(http://usa.yamaha.com/products/music.../?mode=model):
"64 MB Flash ROM for loading custom WAV/AIFF files...Maximum size Mono sample: 2 MB Stereo sample: 4 MB...Built-in pads 12
External inputs 5 (three-zone x 1; monaural x 4)"
Note that in addition to much less overall memory, the Yamaha also has a per sample
limit of about 22 seconds. Could be limiting for backing tracks.
For backing tracks, or more and better drum sounds, you could probably use either unit (or older, cheaper, simpler units) as just a MIDI controller, hook it up to some far superior sample player/synth (e.g. a laptop with soundcard) and get whatever options you want. Way back in 1989 that's what I did. I still have my Ocatpad-II, although I'm not sure if it even works.
With the spd-30, you can layer two sounds,
- that seems kind of lame to me in 2013. My Ocatapad-II from 1989 had three layer mixing or switching.
I seriously looked into both of these units some time ago, before deciding to get a full electronic kit, the Yamaha DTX-562. For me, I wanted something I could play like a real drum kit, but a real drum kit is out of the question for noise reasons, and you get some more flexibility and functionality with an e-kit.
If I had to choose between the SPD-SX and the DTX12, it would be a tough choice. The SPD-SX has a LOT more memory (which would probably sway me), but the DTX12 has more pads and more pad expandability and appears to be slightly cheaper.
you create the sounds on something else and then transfer the sounds to the unit.
- I would generally recommend this approach. I used to have a sampler, and it was a real pain. It is much easier to use a DAW to create WAV files and then download them. "Sampling" sounds so easy, but, if you're trying to (re)create some kind of instrument, it is a lot of work to get good sound. If you're just recording a chorus, that is an easier task. In any case, having a full computer display, mouse and keyboard to edit on is really nice.