I am a registered GS3D user, but once I tried GS Arcade that is pretty well all I have used. It is much more user-friendly and has a nicer interface, including chat, but none of that is particularly significant to me, since I have had GS3D set up to my liking for a long time (though GS Arcade's auto-updating is handy).
I mainly prefer the way GS Arcade quickly polls my servers, starting with my favorites. I find I am able to get into a game much quicker and with less hassle. I still have GS3D on my system but virtually never use it.