hi
I don't know the FS versions....I'm coming from a FE Backgournd...But I assume the power delivery is much the same.
I ride a 650, I thought that it was going to be too much for me. But after getting used to it, you realise how friendly it is, No Suprises, The Throttle repsonse is so perfect it feels digital...They say that the 550 is quicker (but not faster, and you're not going to do hare scrambles anywayz), but i feel the power on the 650 makes it easier to ride, you can just slip it into 3rd and ride all day.....and specially in street trim, i think the 650 will smoke anything you can throw at it!!!! NO DOUBT, Go for the 650...
Weight, Handling, setup is exactly the same on the 550 and 650.
If it's speed you want go 650. i did a run with a GPS in Stock trim, on a 200m long bumpy dirt road, and reached 160km with a gear to spare.
SOOOOO GO 650. (IMHO)