I would just get the XL. From what I have read on the heli forums the XL has a lot more power than the L and it's not much more money.
I think the problem I had is because of the 600XL being an outrunner. I guess they are harder to start up than an inrunner. It must have been too much for the MM with the starting power on high. I sent an email to Castle about a month ago asking if an outrunner would be a problem and I never got a reply. I guess I figured it out myself.