I have owned both the FR2-LE and the 702. The SD is clearly a more impressive, solid, pro piece of gear, but I'd say the audible differences are minute. The difference in the advertised EIN (Equivalent Input Noise) is only a few dB, distortion is well below audible limits for both, and they're both flat 20-to-20, the rest is for cork sniffers.
What will make a huge difference in the final result is the operation of the boom, selection of location, cooperation of the crew. If you've got a quiet spot and a quiet crew and put the mic in the right place, you'll get your sound with your current gear. And if you don't, you won't get the sound with the SD and Senn.
Fran