I've got two N-gages in hand and one GBA SP.
Trust me. Get the GBA. The N-gage is going to be one of the biggest flops in gaming history.
Also en route is a new device called the Zodiac from Tapwave (
http://www.tapwave.com), which is a combo Palm/gaming device. It'san excellent piece of hardware with real world functionality, but the game selection will be limited at launch. (Well, actually, it'll be compatible with every Palm game ever put out, but it'll be short on games specifically written for the Zodiac.)
If you go the GBA route, I have yet to meet anybody who hasn't LOVED "Wario Ware" - no matter what genres they usually like.