I have the Yamaha DTXplorer 2, and personally, you should go with something of better quality.
The problem with the DTX is the pads aren't very sensitive and you'll get a lot of misses, not to mention they don't feel good. Also, something's up with possibly the MIDI bandwidth/polyphony on the audio module because you'll get misses when a lot of hits are triggering at once.
You're far better off with a Roland, but you'll pay a lot more.
I'm not sure about the Alesis, as I haven't played one, but if you get a chance and find one at a local music shop, you should try it and compare to the DTX.
Also, if you see a Roland kit with mesh pads, jump on that and give it a spin. You may find yourself considering saving your money for a Roland. They're worth it.
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