Is there a big improvement upgrading from 90 cm to 100 cm dish?

I don't think so... With a 100 cm. dish, the wind will cause more problems.. What you need is a good solid mount, that will not move with the wind.. I would pay more attention (and money) on the mount, not the extra 10 cm dish.!! (Also make sure there is no 'play' in the motor action..)