THINGS TO CONSIDER WHEN BUYING A PORTABLE KEYBOARD
The best thing about portable keyboards is that they are... portable. Which means that you can use them to practice or even perform virtually anywhere - if they have the right features which include being battery powered, and having manageable size and weight.
Number of Keys
Most portable keyboards come with 61 keys, since it allows for two-handed piano-style chords and arpeggios, while keeping the bulk and weight manageable. Those with more keys allow for even more notes to play with, but expect them to be heavier, bulkier and not to mention more expensive. Their bigger profile also requires you to prepare ample space for playing and transportation. There are also those with fewer keys which are cheaper and easier to carry, at the expense of limited notes to play with. The 61-Key configuration sits nicely in between the big and small keyboards, allowing for good access to notes while keeping the instrument reasonably portable.
Key Size, Action and Sensitivity
Most portable keyboards come with semi-weighted synth action keys that balance dynamics and playability nicely. If you prefer the feel of an acoustic piano, then having a full-size, weighted keyboard is ideal for spacing and finger tension consistency. But be warned that keyboards with this feature are heavier and usually more expensive. Touch response and sensitivity are features that separate toys from a real instrument, so they should be considered. These features allow for volume changes based on how you press the keys, which can be used to better express your music, much like an acoustic piano would.
Voice, Style and Rhythm
Portable keyboards usually come with as many sounds that the manufacturer can put on them without jacking up the price too much. As such, some of the voices or styles may not sound good to professionals, but they can be useful and fun to play with for beginners. Some even come with effects so you can personalize the sound better, or mimic the sounds used on your favorite songs. Most keyboards also come with drum and percussion sounds, along with pre-made rhythms that can be used to accompany you as you play, again a nice little feature for fun use, and more importantly, for training to improve your timing.
Recording and MIDI
Some of the latest model keyboards allow for quick recording, either direct to the keyboard itself via a memory card, or on to a computer via a USB cable. While not necessary, recording can be a useful tool for practice and for songwriting. MIDI connectivity will let you use the keyboard as a controller for your MIDI device or virtual instrument software. The most common MIDI connection these days is USB, but there are still keyboards that have 5-Pin MIDI sockets.
Mod Wheels and Other Controls
The more control options there are, the more use and variation you can get out of the instrument. However, the drawback to having more control is that you may end up wasting time tweaking than instead of actually practicing or making music. Most students will usually just need to play traditional piano parts on the keys, but if you are into modern electronic music, you'll want mod wheels and other extras.
Student Friendly Features
I found that most of the keyboards have a number of student friendly features which include metronome, auto accompaniment, headphones out, and some even include virtual "lessons". While these won't take the place of an expert mentor, they are valuable for someone who wants to hone their playing skills.