The biggest difference is the six speed gear box and the clutch, neither of which I really use. The build quality is better and you have the steel pedals and wheel and the leather. But in terms of difference in feel in game between the Momo and the G25 the difference isn't massive.
What the G25 allows you to do, that the Momo doesn't, is simulate driving a Mondeo. It has 900 degrees of rotation, a clutch, leather steering wheel and a six speed gear box and a push down reverse.
The trouble is, non of the games you play are simulating driving a Mondeo around town, so all of those feature are kind of redundant.
So if your Momo is still in good working order then I would say stick with it.