I'm not a drummer and I don't claim to be. I'm a guitarist. But given I write and love to record my own music, I need a drummer. I also live in an apartment so being super loud isn't an option. A drum machine fills this roll. I use it to program in and play drum parts which I then record and overdub to. It saves me the hassle of having to be talented in the drumming aspect and I can lay things out quickly, record in studio quality, and yea... just a lot easier.
For me, either one of these units does the job. First an overview of what each does well.
I think Alesis is the better choice if you are using it live. 3 things it offers over the Boss Dr-880
Roll: This button allows you to roll any note to the quantize value of note you've selected up to 32nd notes. This is absolutely awesome for dropping in live drum solos and just fun to have access to in general. This is the only real huge seller for me over the Boss DR-880 and I can't believe they didn't include this functionality. It seems so simple.
A/B Fills: Alesis has the ability to program in fills for each pattern which is pretty awesome, but not necessary. I can get around that in my situation since I'm not using it live.
Variety: Alesis seems to have more variety in the way of drum sounds.
Number of pads: The alesis has 12. Boss has 20. You do the math. A couple things I love, the Boss unit includes 4 toms, the alesis does 3. I love using toms for descending rolls. More variety = more fun. Both units include snare, but the Alesis programs one of the snare parts to sound like a cross stick (I think that's the term) where the drummer hits the stick across the metal rim, not the drum itself. So instead of having a good left/right snare you get 1 snare. You can get around that by programming your own kit but still, annoying. The Boss unit also has 2 crashes, that is important for me cause I mostly play hard rock/metalish type music and so a variety of symbol crashes is key to the music I make sounding half way decent.
EZ Compose: This makes it super easy to drop in a drum beat for practicing. Genius in my opinion.
Ghost Notes: These make your parts sound more human like
Groove Modify: You can add fills, ghost notes, swing feel to make things feel more human like as well
Touch Sensativity: Alesis loses here. It basis touch sensitivity on "Ranges" It has 0 - 9 levels, and you can't choose to use that whole range. You choose a range (soft,medium,loud) and it picks around 3 of those value to bass your touch sensativity on, leaving you with 3 levels no matter what. Bosses differs from 0 - 127 and you can use that full range. The difference is clear. You'll here A LOT more dynamics on Boss's unit. It sounds much more realistic.
Layout: I think Boss made the layout a lot more logical than Alesis's unit.
Multi Effects: I'm a guitarist. Of course I'll take a bunch of Boss guitar effects :)
Volume Levels for Each Part: I can control the volume level of my guitar, bass, and drum parts. Also individual outs. Great for sending to a mixer or other recording device.
DVD Manual: I hate reading. You can pick up a walk through by proaudiodvd's which I highly recommend.
Time Signatures: This unit actually does different common time signatures, which is awesome as I like to switch things up. The Alesis unit kinda does, but offers it more in the how many beats per measure category. I think it does up to 24. I'd really like to see one of these offer the ability to create your own signature.
Drum Sounds: I think either of these units has great drum sounds. I really can't choose between the 2.
Outputs and inputs: These have all the outputs and inputs I could ever require, anything from Midi, to TRS L/R, to foot controllers to control patterns. Boss does have a USB connection as well. Alesis does not.
Durability: Both are solid units. Alesis is smaller, Boss is bigger.
No one wins!
Both units pre fill your user patterns with a bunch of samples. The SR-18 is the worst. It fills the first 100 "preset patterns" which you cannot change, then 75/100 of your "user patterns" with their own crap. The worst part is that the user patterns are all individual, so you can't change them without losing them completely. This will leave you in the constant debate of keeping them or wiping them out for your own stuff. If you want to put in patterns cool, put them in your own section. Don't eat up mine.
Usability: Both units aren't super hard to pick up, but take practice and concentration. I believe this is the reason that some may go straight to the computer to program beats as things are probably a lot less complicated.