The seat height is only 0.3 inches higher, if you wanted you could get an SR and it would be 1.1 inches lower. On top of that you'd get better on road handling, I don't know why but a lot of people seem to automatically go for the DS/R when they're only riding on road.
0-60 numbers on the English Electric site have the FX and FXS at 4 seconds and the DSR at 3.9, the SR at 3.3. I don't know if there really is that much difference between a DSR and SR (if they can both be launched at full throttle, so traction isn't an issue on the DSR, then they should be very close to identical) and I find it hard to believe that the FX/S is 1.2 seconds faster than a 7.2 DS when they have the same power and torque and the DS is only about 10kg heavier. Also they say the 7.2 S is 0.4 seconds faster and I know for a fact I can launch my DS at full throttle.
Have you considered the weight different between your FXS and the bikes you're looking at getting? The DSR is going to be a tad over 50kg heavier and the SR/F is 85kg heavier. I would say look at a 7.2 S as it's only about 10kg heavier, has enough range, has more storage, and will have faster charging than your FXS (and is of course quite a bit cheaper), but if the numbers above are correct it will end up being a bit slower to accelerate. You will however have a higher top speed at 98mph compared to 85mph for the FXS (unless you're in the EU).