I've got a 1st gen G15 on my i7 and a G110 on my i5 HTPC. I like them both about the same due to their different features. The folding screen on my G15 is wicked handy for some games and when I just wanna keep an eye on my CPU activity. The Old G15 had 18 Gkeys, and not just 12, which isn't really that big of a deal after all. Simply having Gkeys is usually enough for me now that I don't play wow anymore (used all 18 keys for diff macros in wow back in the day...) I also own Logitech's G35 headset which is a USB interface, so the G15's 2 accessory USB sockets work out better vs the G110 which has one USB socket and then two analog 3.5mm jacks for mic in and headphone out... but that could change with a different headset.
Also, the G110 has a crap load of different LED backlight colors you can choose from (shades of red, blue, and purple, or any combination of the three) so my wife actually uses Pink lighting when she's typing... not a huge deal, but I like that she likes it... the 1st gen G15 (don't know about the current ones) only has blue lighting.
As for the typing, The G110 is quieter, but that could be because my G15 is almost two years old now. =)
What ever you do, don't take a G15 apart in order to clean it... it took me near two days to put the damned thing back together due to all the different cables and LEDs.