I'm also in that situation. I've been running AVR 125 for the last three years and it has served me exceptionally well.
The reason I am considering upgrading to the AVR 240 is because I'm finding it hard to mate the Axiom M3Ti Fronts I have to the JBL E150P subwoofer that I just purchased. The Axiom M3Ti can reach down no further than ~85 Hz. From the suggestions of many, the consensus is that I should set the M3Ti fronts to "small" and use the receiver x-over to send the low stuff (L/R +LFE) to the sub. The trouble is that the AVR 125 has only one x-over at 100 Hz, which clearly does not work well in my situation.
Regarding the AVR 240, the benefit is better bass management. The x-over points you can choose on the AVR 240 are 40, 60, 80, 100, 120, 150, and 200 Hz. In addition, you can set the crossover points on the fronts, centre, and surround(s) independently of each other. Throw in an OSD, higher wattage and 7.1 capability and I'd say you have a keeper.
It really depends on your needs. If you just have a 2 channel system, then the 125 is ALL you need.
Hope this helps.
AJW