Sorry to deviate off topic for just a moment here, but SafetyBob, why are you even trying to shoot playing kids with manual focus cameras anyway??? Clearly there are better tools for that task. Your 50% success rate makes that obvious enough. Add composition, timing and lighting issues onto that and you must get very few pleasing results. And yet you continue to throw away 50% of your film after paying for it and processing it. What could possibly be motivating you to do this? Do you ever use AF cameras?
First of all, don't sell your Leica kit. You will always find a rationale for your M2 down the road. I have a large assortment of medium format equipment plus my M3 with assorted lenses. Although it doesn't get as much use as before I stepped into MF, I picked it up last week and renewed my acquaintance and had a blast shooting quickly and unobtrusively. Now, regarding which one to get--if your main rationale for moving to 645 is to have lighter rig to carry out--suggest you handle one of the above first. I have a 500C as well as a Mamiya Super. The Mamiya with prism and motor grip ways about as much as the 'Blad and is virtually the same size. Your main difference will be in how you shoot. You'll probably be moving from a waist level finder on your 6x6 to a prism with your 645. That will change how your compose frame your subject. This will be your most overriding concern as you consider changing equipment. You might find that for landscape, a WLF is better for style of work.