FRA is one the airports I've flown the most into/out of so I've spent a lot of time around there. I agree the Westin is the best SPG property in the area but for one night I agree with UA-NYC to stay near the airport so you can get to both Mainz (1st choice) and downtown Frankfurt (if you're done with Mainz).
I prefer the Sheraton Congress because it has a nice pool; a vigorous swim helps with jet lag. (The Westin is the only other SPG FRA property with a pool.) It usually has the best rates of any FRA-area SPG hotel but offers great Plat recognition, as seawolf mentioned. It doesn't have a lounge but I use the LH lounges at the airport instead.
I like the Sheraton FRA for the true airport (and its train station) convenience. It has one of the nicer Sheraton lounges I've seen. I've never gotten an upgrade beyond a Club/Tower room, not that I care for a 1-nighter. Both are really good about early check-in/late check out, but sometimes the Sheraton FRA check-in lines are so long a taxi to the Congress would've been faster.
I haven't seen any reason to try the Element at FRA. If I need to take a bus or taxi I might as well as go to the Congress, which seems to usually have similar rates. The Congress has a tram stop in front and S-Bahn station nearby so it's almost as good as FRA for transit access to the area; no such access from the Element. If I need to be at the airport, might as well stay at the hotel where I can walk to/from the airport.
Thus its Congress for value, Sheraton FRA for convenience, and Westin for upscale experience.