What are the specifications on the iPhone accelerometer? If this were something I was working on, I would start with a static 3d test to validate the imu output when it was fixed In multiple known positions. The iPhone uses a fairly cheap part, you could probably figure out what the part number is by doing some sleuthing, and then you could figure out if the iPhone is better or worse then the imu's you are looking at
From this teardown http://www.ifixit.com/Teardown/iPhone+4+Teardown/3130/2 Apple seems to use a ST Micro proprietary part. This part ST L3G4200D (http://www.st.com/web/catalog/sense_power/FM89/SC1288/PF250373) is mentioned as a potentially commercially available part.
The Iphone isn't intended as an IMU, and so if I were trading off performance vs cost when designing it, I'd go for the lowest performance, cheapest part that would work.
In your case, I'd build a fixture that I could mount an IMU in, and fix it in multiple orientations so I could determine how accurate and repeatable the IMU reading were in a known, accurate position as you're taking data. Make it so that you could put in arbitrary positions and you can evaluate coupling between directions.
I like having measurement techniques that are accurate enough to truly evaluate differences between systems. If you're using a iPhone as your reference, then you'll probably have trouble figuring out the difference between multiple sensors.