Polyphonic (or "key") aftertouch means that aftertouch information is associated with each note that is played. Monophonic (or "channel") aftertouch means that aftertouch information is sent out per MIDI channel and applies to all notes sent on that channel. To get two notes to bend different amounts, polyphonic aftertouch is required. However, as Nigel says, there is a lot that can be done with monophonic aftertouch.
Due to the added mechanical complexity, not many keyboards support polyphonic aftertouch. I am not aware of any in production. However, it is not hard to implement electronically so many modules and keyboard synths will respond correctly to polyphonic aftertouch. Keyboards that generate polyphonic aftertouch include: Roland A-50, Roland A-80, Kurzweil MIDI Board, GEM S2 and S3, Ensoniq SQ1 and EPS, Yamaha DX1, and not very many others. It's a shame, because polyphonic aftertouch feels very "natural" and is the most expressive controller available in MIDI.
However, there is another issue that affects the usefulness of aftertouch for pitch bends, and that is the resolution of the aftertouch. Aftertouch resolution is apparently not critical when aftertouch is used to control vibrato, etc. However, to emulate a guitar, smooth pitch bends are required. I am not sure that even 128 levels is enough and I seem to recall some synths implement a sort of double-precision for pitch bend information. On the other hand, many keyboards that implement aftertouch do not even provide 128 levels. This is the problem with my PSR8000 and it does not do pitch bends well. I have been able to get the effect I want on my A-50 driving my XV-5080. Some of the patches on the Roland Fantom use aftertouch efectively to control pitch bend.
The moral of the story is you cannot assume that because a keyboard has aftertouch that it is suitable for controlling pitch bend. Try before you buy.
[This message has been edited by Clif Anderson (edited 01-14-2002).]