I'm never going to tell you not to go to a water-saving model, so let's talk Gerber!
I double-checked, and here's the piece that lays out the UltraFlush models:
http://www.gerberonline.com/downloads/productliterature/fixtures/Ultra_Flush.pdf
And, upon further research, here's the deal: You are correct to go for the 21-325, as it is the ADA height you want with the rear discharge. However, the question is which version to get: the 21-325, the EF-21-325, or the DF-21-325.
My advice would be to AVOID THE DF VERSION. I checked the pressure-assist units, and it appears that WDI is still the manufacturer of the dual-flush. You just don't want it, based on all the comments on this forum over the past few years. WDI used to have a troll that would get on here and say that we were wrong and it was fixed, blah, blah, but the reality is, complaints still came in. The FlushMate hasn't had these issues, and the exploding FlushMate IIIs have been replaced in the last 5 or more years by one that has sold hundreds of thousands of units with no problems.
If water-saving were my goal, I would buy the 1.1gpf version (the EF), because that should be sufficient to flush anything meaningful. Both the 1.1 and the 1.6 are official Sloan Flushmates, only the dual-flush is the WDI unit. At 1000+ in the MAP testing, I think you're not going to have an issue. However, if you have family members who clog the bowl regularly, then I would get the 1.6, and be satisfied that I had done my part to help reduce water usage.
But I would avoid the dual-flush on this toilet.
Shop around because delivered prices are going to vary widely, even at different plumbing suppliers in the same town. I would call and ask for a price on the specific model number you want, in the color you want. Pull the yellow pages and make yourself call five places. You will be surprised what you find.