The 3500 is only 90 euro more expensive, gues it has more features ,more sensetivety-better specs and is quicker, i own a Icom-R-20 ,but if i had to spend my dime again ,it would be the 3500.
Ps- A good antenna would be the D-777 - it does a good job both with the 125 and 3500 !!!!!