1) the difference between the two levels of available output is about 2dB.

2) your speakers are moderately efficient with a sensitivity rating of 90dB.
...(hopefully that is the truth)

3) the 7125 is weighs less for shipping purposes

Based on the above, I would try the 7125 first. If within the 30-day trial period you find the 7125 is not quite enough and you wish to trade-in for the 7500, you’ve lost less in shipping that if you tried the 7500 and wished to trade-in for the 7125. Of course if you go with the 7500 first, you probably won’t want to give it up. I chose 200wpc because my speakers have a sensitivity rating in the mid eighties, so right away you’ll have more overall acoustic loudness available at 125wpc that I do at 200wpc. Which brings up a point in favor of the 7500 if you are going to keeping whichever amp you choose nearly ‘forever’ – if you ever change loudspeakers to some that are several dB less sensitive, you’ll have power in reserve to drive them harder just to achieve about the same acoustic level as a 7125 matched to your current Polks.

Rather than trying to match a speaker’s power rating – a power rating of 200 watts means it could stand that much input for a very short duration without destroying itself and likely you wouldn’t want to exceed a continuous feed of more than 40 or 50 watts anyway as this might cause a slow buildup to an overheat condition – the 7500 allows a little more headroom for spikes to cleanly be reproduced, so that when you are listening loud and detailed spikes occur, the sound remains very clean sounding. That is one of the distinctive differences I noticed between my 100wpc receiver and Outlaw separates when I upgraded – the Matrix was a hot DVD at the time and the gunshots were quite clean and crisp sounding via the Outlaw gear while ‘mushy/hashy’ through the receiver. If you forsee keeping the Polks as long as the amp you purchase, then, because the speakers are moderately sensitive, the 7125 will be cruising on a small percentage of its overall capability and you'll have that 'clean' sound from the 7125 as much as from the 7500. Additionally, with the 7125, you have the opportunity to easily either go to 6.1/7.1 later on or bi-amp your mains.

And then there is this: if you do wish to buy a newer processor/amp combo in the future, which combo would be easier to sell, the 990/7125 (a 7.1/7.2 system at 125+ wpc), or the 990/7500, (a 5.1/5.2 systen at 200+ wpc)?

Bottom line: there is no clear winner. The deciding factor may be the finer points of your current priorities and your best guess about your listening future.