Most of the other 16bit-to-GBA ports are enhanced ports, with new stages, bosses, dungeons, dialog, and what have you. From what I can grasp, this isn't as much of an enhanced port ala the Super Mario Advance games, but more of a direct, simple handheld port?
Nope. Think of it as Sonic 1: Drunk Edition. The screen is cropped, the game runs at 75% speed, music is remixed (badly), some behaviors for some props were changed, bad sound effects, spindash doesn't protect you, backgrounds lack parallax scrolling (like the NTSC/PAL versions, except Green Hill Zone is like the JAP version, Starlight Zone has no parallax layars so the graphics are worse,) and it's not very fun to play. Get the PSP version (Forget the title, contains Sonic 2) or DS version (Sonic Classic Collection, Contains Sonic 2, Sonic 3, Sonic & Knuckles, Sonic 2K, Sonic 3K.) Both have Sonic 1. The only other version I want you to avoid is the Game.Com version, but those are your portable options. I say get the DS version.