I own an Oxygen 49 and mainly use it for fooling around on my laptop while watching TV. My only real complaint about it is that the velocity sensitivity is hit or miss on a good day and generally I end up simply having to turn it off entirely.
Otherwise, looking at the specs on the Alesis I'd have to say if you can pick it up for the same price it is probably the better buy. If only for the fact that it has real MIDI ports on it as well as USB. The Oxygen is USB only. The MIDI ports will become useful if you someday opt to connect to a hardware MIDI sound module.
Then again, the Oxygen is USB powered so one less cable and wall wart for powering it up. The Alesis uses a separate power supply.
Finally if your main interest is recording drum beats, the Alesis has dedicated drum pads while the Oxygen does not.
If you can pick up either for the same price, I'd have to say the Alesis is a better choice for you unless the additional power supply is a dealbreaker.
Gary