In terms of cost I would also consider the price of electricity as the 6610 uses about 50w+ or so than the 7250. 50w*24*31/1000*.17 (average CA electricity cost) and you're looking at about $76 more in energy cost for the 6610 per year. Not to mention cost of removing that heat if necessary (ac).I did consider the 7250, but the price of around $300 (compared to about $150 for 6610) was main reason. When buying used equipment for 24/7 home use, I usually purchase another similar or same model for a spare. I couldn't justify twice the cost for two 7250's.
I agree that modifying power supplies isn't a good idea, and the 40G ports didn't make any difference to me, but the extra 8x 10G breakout links and price were deciding factors for this purchase.
I guess I can attempt the fan mod without opening the PSU(s) and see how temps run.
However, it sounds like the 6610 is totally reasonable if you're using the 40GBe links to breakout to 10GBe. Sounds like you've got a lot of 10GBe gear.