I know there's a lot of stuff out there about this new St. Croix spinning rod.  I am using 6 # yo-zuri hybrid line, the techniques i am mainly fishing with are shaky head fishing, weightless senko, dropshot and wanted to know what rod would be better the 6' 10" Medium Light Extra Fast or the 6' 8" Medium Extra Fast. Would the 6' 10" rod be used only for specifically dropshot? or could I use that for the other things as well and be better off with the 6' 10" rather than the 6' 8

Both would be fine choices.  The 68MXF may offer a little more versatility but the 610MLXF is a great rod too.  If the focus was mainly drop shot, get the 610, if drop shot is secondary to other things, get the 68.  Hope this helps!

I have a 6'8" MXF in the Avid X line, and it is pretty much the perfect spinning rod. I use it for weightless plastics mostly, esp. senkos and flukes, but it handles tubes, grubs, shakyheads, ned rigs, topwaters, jerkbaits, and even drop shot like a champ (although I'm not much of a dropshotter).