I own the SR-125's and have tried the SR-80's briefly for about 15 minutes. In terms of sound, if there were any real differences, I certainly didn't hear any. In all honesty, I believe what you're paying for is the 1/4" plug and better driver matching.
I'd just get the SR-80's since they have the 1/8" plug which will be more flexible. 1/8" > 1/4" adapters are much smaller than 1/4" > 1/8".
I haven't heard the SR-60's, but from what I understand they are the same except they have different pads. Some one will have to correct me on that.
All the Grados will be driven well out of the ipod, but the improvements in sound you are paying for with the SR-225 and higher don't really become apparent because the amplified signal is the limitation. The SR-225's should sound pretty similar to the SR-125's, SR-80's, etc straight out of an ipod, but once amped the differences will be apparent.
If you are using an unamped ipod, the SR-60's or SR-80's are the best choices in my opinion. They have an amazing bang/buck ratio and have 1/8" plugs, negating the need for adapters.