I'm trying to debate this as well.
The nice thing about the AT&T insurance is that it covers if you lose your phone or if it's stolen. The bad thing is that it is $6.99 per month + a $125 or $199 deductible (depending on what tier they place the 920 in) for each occurrence.
Squaretrade is just an extended warranty. The 40% off seems pretty nice though, but they also have a $99 deductible for all repairs (lost or stolen phones not covered). Would pick this if you don't think you may need to get the phone serviced too many times after the warranty is up.
Microsoft Assure is an extended warranty as well and costs $99, with a $50 deductible, unless defects in materials or from normal wear and tear (I'm assuming the normal wear and tear and defects and materials would cover you if the touchscreen doesn't work too well or if any of the buttons lose their responsiveness/functionality, as opposed to dropping it and there being a cracked screen, which would require a deductible?). Also, lost or stolen phones aren't covered under the MS plan.
AT&T Insurance - get if you are one of those people who occassionally loses their phone
Squaretrade - get if you don't think you'll need much service beyond the normal warranty but would like that extra coverage to bring you into your next equipment upgrade
MS Assure - get if you anticipate having to get your phone serviced a couple of times outside of the normal warranty.
I think I may be leaning towards the MS Assure plan, as it seems to cover normal wear and tear (if buttons don't work).