It all depends on your genre and what sounds you are going to play. If you play mainly piano sounds, then 88 key may be most important for the range. If you are playing electronic music, 61 keys are fine. If you are triggering loops from a notebook, 49 keys is more than sufficient (25 or 37 keys is probably sufficient). However, if you are using softsampler with key-switches, you may need 88 keys as key-switches are programmed at the lower octaves (or you'll need a second keyboard - 1-2 octave just for key-switches).
The other factor is weighted vs unweighted/semi-weighted keys. Some people prefer weighted (esp classically trained) and some unweighted (esp those playing mainly organ/B3 sounds). For a keyboardist, we should be able to switch between the 2 without problems. My only problems is playing extensive glissandos on the B3 using weighted keys.
76-keys give you the most balanced range if you can't decide.
Remember that 2 things contribute to portability - size and weight. Midi controllers will weigh significantly less than workstations and most synths. Roland has some really light-weight synths (some Juno series). Otherwise, an 88-key midi controller + notebook/softsynths/softsamplers may be pretty portable.
Or, if you have the money, you can always go for the most portable solution - Infinite Response foldable VAX77.