there's quite a few differences its not just 1 has a 650 motor and the other has a 1000, front forks different, swing arm slightly different, 650 has cable clutch 1000 has hydraulic, there's quite a few odds and ends different.
Now which one you want is down to personal preference .....how you like to ride? two up alot ? or just back road twistys ? I have had 3 strom 1000s and 1 650.
The 650 is a very happy bike, great fuel figures, great round the back road twistys will do everything you want it to, but for me I just missed the pure grunt of the 1000, like I said its all down to what you want from a bike .