In the end, I think it's up to you.
I know people that run a 120/90 in the front and prefer it over the 110/90.
Those people also say if you only do mild street riding, much of which is in a straight line, then the 120/80 is fine.
But personally, I would go with the 110/90. My bike came (used) with a 120/80 and I wasn't able to lean the bike as easily. You have to do more body leaning, rather than bike leaning. Also, it doesn't feel as stable when you actually have the bike leaned over. I didn't use the outer 10% of each side.
Furthermore, since the 120 was squeezed onto a 2.5" rim, it wasn't evenly placed around the wheel, and at 80+ the bike would shake violently.
After that tire wore down, I couldn't find a 110/80, so I put on a 110/90. Now I can ride the bike the way it was meant to be ridden, it doesn't shake at high speeds, I can lean it over more, and it will turn quicker.
As for the rear, I have no experience messing with tire width, since I've always had a 130/90 on the rear.
How does your bike ride now?
Do you like to lean it over?
Are the 120 and 140 the only tire sizes you've experienced on the bike?