I would opt for their 50wt cotton in the bobbin. My understanding is that using a cotton thread with a slippery thread helps maintain a balanced stitch, as the cotton "grabs" the other thread. Plus, the two threads would be closer in weight to each other which also helps.
I'll have to look on the Fil-Tec site. Didn't know they offer a 60wt silk thread! I'd like to try it too. Please post how you liked working with it!