I don't own any of these handhelds but I have seen and played the NGPC.
For the record I do own a GBA/SP and a Nomad.

A couple things that I really liked about the NGPC was the way the D-Pad/Joy stick fealt and the fact that the NGPC has a system OS of sorts with the calendar, world clock and horoscope. And much like the GBA/SP, it fits in your pocket, bonus points right there!