To help you choose a keyboard amp, Let's examine the conditions that most keyboardists face in live performance and build a list of requirements based on those conditions.
Sound Reproduction: First of all, the frequency range of keyboard sounds is much greater than guitar so you'll need a full-spectrum frequency range to reproduce your sound accurately. This automatically disqualifies using a guitar amplifier. You might be tempted to say, "well, it's got a 12 inch speaker and an amplifier section, like a guitar amp, so what's the big deal?" First of all guitar amps are designed cut bass response, have a presence boost in the midrange, and don't go much beyond 10kHz in the high frequency range. In essence, your sound will be squished into the guitar's frequency range, which means you'll be fighting for sonic space, which will cause the inevitable volume battle and less clarity. More importantly, guitar pickups output a low voltage signal, which is why guitar amps tend to have a preamp circuit. Keyboards usually have hot outputs, so having a preamp circuit will just add noise. Keyboards also have stereo outputs and you may wish to take advantage of that. A guitar amp has mono inputs.
Portability: Do you mind lugging around a big rig? This might not be a problem if you have roadies, but for the rest of us, size does matter.
Power: The more watts an amp will produce the louder you can play without distorting. Of course, the number of speakers and size of the cabinet they're in will play an important role as well. If you're in a band that likes to play loud, you have two possible options. Use one of the bigger combo amps to match the other players' volumes, or opt for a smaller amp for use as a personal monitor. If you choose to go this route, make sure the keyboard amp has line outputs. This allows connection to the house mixer, which puts your sound in the mains and the monitors. Line out is preferable to miking the amp since the direct signal will most likely yield the best sound.
EQ: Keyboard amplifier EQs can be anything from simple tone controls (treble, midrange, and bass), to a multi-band graphic EQ. Using an EQ to shape your sound in a particular room is best left to the soundman. Generally graphic EQ is used to find problem frequencies in a particular environment, and cutting a frequency band may be Okay for one sound, but it may ruin another. The same holds true for EQ-ing your amp. Since EQ is a global thing on amplifiers, you may EQ one patch to sound good to the detriment of another. The best use of EQ would be if you are using a keyboard amp as a monitor: Roll off some bass and boost the highs a little for clarity. So in essence, a bass and treble control is pretty much all you'll need. Also keep in mind that the samples in your keyboard have already been EQ'd in the studio to be optimum before they're loaded into your synth.