I'm just back from 3 weeks in Catalunya on my new Sherpa frame, same size 610S, I had the same uncertainties about frame size between the 585L and 610S but am happy I went for the 610S, it is a better size for a taller rider (I'm 6'2"), and the top tube length difference is minimal when you take into account the different seat tube angle. Top tubes differ by 15mm, seat tube angles differ by 1 degree, so on the 585L the top of the seatpost will be about 10mm further behind the bottom bracket, so for the same seat to pedal relationship your saddle would have to be 10mm further forward in the clamp leaving just about 5mm effective difference in top tube, which is pretty minor and can be accommodated by stem length.
Hope that makes sense, not sure I've described that too well!
Here's a photo of my Sherpa, haven't had time to cut down the steerer tube yet, had to order the frame a week before going on holiday due to the old bike giving up the ghost at short notice! Thanks to Andy etc at SJSC for the quick service in getting me the new frame ASAP.