Samsung galaxy s2 I9100       
1.2Ghz dual core Exynox 4210. 
Overheat easily.                       
Poor battery life. (Fixed with Android 2.3.5)
Fully run at 1.2 GHz CPU.
So far no apps/ games compatibility issue reported.
Better 3D performance.
Lot of custom firmware, kernel, modification, tweaks.
Normal text font in PAPAGO.
Perform better and less lag while browsing the Internet and score better when opening apps/multi tasking.

Samsung galaxy s2 I9100G
1.2Ghz dual core TI OMAP4430.
Doesn’t overheat easily.
Better battery life.
CPU capped at 1008 MHz.
Might have more apps/ games compatibility issue.Some games developed for GT-I9100 like Asphalt 6 can’t be launch on it.
Poorer 3D performance.
Not much custom firmware / modification.
Very small fonts size in PAPAGO
Might feel lag when opening up too many app compare to GT-I9100 model.

With reference from http://10division.com/samsung-galaxy-s2-gt...u-need-to-know/

Sometimes we need to leave it to other to decide for it. But truthfully said, S2 non g more ppl wanted to buy.

not really old stock... depend on how... Me here still can find new stock non-g...  

i hate that G, but nevermind i still wanna buy it tommorow. cannot tahan!

i am not agree with u... i think the G version is better since it compatible Gameloft games is more than non G version...
non G version use Mali-400 which is not common platform are less compatible gameloft games.. and G version which use Power VR GPU which nowadays common platform are support many gameloft games. same with the new galaxy Note which using Exynos cpu also faced this problems.... Many previous user complain that non G version got game issue.. like laggy on game, cannot open the game and some else... that's y Samsung change the CPU to OMAP 4430... see what u need... playing game or just simple use.. For the gamer choose G, otherwise choose non G for simple use....

The G version can't use custom ROMs?

Objection! Your statement are wrong!
http://forum.lowyat.net/topic/2068824

This is only the problem of time. Maybe SOME custom roms, kernels will be created for G version in the future.
There are also people who never play custom roms. G version is better for them because of improved overall stability of the phone.

Oh, my bad..
I9100g - omap 4430
Nexus - omap 4460