The 50's are a very nice speaker and to me seemed liked the produced a little more bass than the 60's. The extra mid driver in the 60's seems to blend the whole sound so the bass doesn't stand out as much. Sorry, it's tough to describe.
You'll be more than happy with either but the "what if" will always plague your thoughts. As for the distance to the back wall a minimum of 4" is suggested. Of course, until you get them in your room and play with placement you won't know for sure. I found mine very easy to place and haven't noticed much of a difference moving them around. If too close to the back wall and the side wall then yes, an increase in bass wgich gets overstated though.
Good luck
