Roland Octapad

Good question! I only have experience of the SPD. The sounds are good and the phrase loop function is nice feature but more for practice and toying around. I.e. for the beginning drummer. From reading a bit about the Alesis it seems to be quite all right. Be aware of how the pads feel though. They might have cross talk and lack of dynamics/sensitivity on these cheaper models. Hard to say for me since I haven't tested it. Any reviews on this?
The spd has good pads and sounds but is also expensive because its Roland. If you are into drum kit sounds mainly you will see that they aren't as many as you might have wished for. There's a lot of percussion sounds on this.
Personally I wouldn't recommend this for the beginning drummer. An experienced drummer/percussionist uses an octapad mainly as a compliment to his/her ordinary kit. Of course, octapads are good if you have limited space and electronic drums are good if you have sound level restrictions but sincerely: Learning to play drums on a octapad you will always later feel awkward playing a real acoustic drum kit.
Learning to play drums should be done (if possible) on 1. an acoustic kit. 2. an electronic kit.
There are lots of used good kits to pick up on the market, craigslist etc. Also remember to learn from a teacher. And get some friends to practice with. VERY important!
But if compact octapads is your need I'd go with the Alesis. Seems all right. Bring a drummer friend to the store to compare the two.
Good luck!