I have both. A '73 750 Roadster and a '73 850 Interstate. The performance is pretty similar, since the 750 has 10:1 compression and pulls a 19 tooth counter shaft sprocket while the 850 Interstate has a 2S cam and pulls a 22. The 750 is a little quicker, but the 850 will run all day at 80 mph freeway.
More important is riding position. I find I must sit half way back on the stock seat on the Roadster to have a comfortable angle to the foot controls. Two up on this bike is a real pain. So much so that I recently added a Corbin gunfighter seat. Pretty much eliminates a passenger but the riding position is perfect.
The 850 Interstate is much more comfortable because the large tank pushes the seat back so the controls are at a better angle (for me). I added the lower, narrower European bars and rearsets and made it a perfect solo or two up ride.
My point is, ride both the Roadster and Interstate and consider comfort before you decide.