*screen is bigger
*removable back/battery/SD card
*stock ROM is more tolerable (though I still use Nova Launcher)
*lighter than G2/feels better/back isn't gross
*rear keys are much improved
For me, it was a no brainer - sold my G2 to a buddy, then used the profit to do alternate upgrade and swap out the phones. Made a little cash and got a newer, free phone. At $199 or full retail I probably would have held only my G2 though. I will say my G3 seems to handle the 3G/4G handoffs a little better, my G2 (where I work, I get horrible, horrible signal strength) would get hot during the day and often I would shut off mobile data to preserve the battery. The G3 will get me thru a day at work even with the terrible connection without shutting data off. The battery life is very acceptable, with a fully day of usage (yesterday as an example on a 50/50 mix of LTE & WiFi I got to just over 5 hours SOT, 16 hours off a charge and had 6% left, and did a fair bit of video streaming during that time) for my needs easily.
For free/cheap, I'd say do it if the size doesn't bother you. Someone else in the thread mentioned it doesn't LOOK bigger, but it FEELS bigger. I'd say that nails it right on the head. At full retail, I'd say stick with what you've got.