If you're going with 4S-6S I would personally choose the 115A SBEC. If you are going 6S your amperage will stay correspondingly low, but on 4S you want 100A+ capability for sure. I recommend the SBEC because I like having a sold state throttle when I use HV digital servos with my transmitter on 11ms frame rate. The main reason for OPTO's are HV units- so if you're going to be =< 6S, I think you'll be better off with the solid state throttle. Just disconnect the red wire from the throttle lead connector, and use whatever form of RX power you want. I usually lift the flap on the connector and slide the red pin out, and just tape it back against itself, that way if I want to use it for a different application that needs a SBEC, I can just slide the pin back in. It's not hurting anything for the SBEC to just sit there doing nothing and the ESC will run cooler. It's always there if you want to try it at the 6V setting. But me, I'd go separate with a castle and set the output to 7.4V or just run a 2S lipo straight through.
-Tim