Well, Ralph, as I understand it, the lift bowl models are inherently more rigid, and so can have bigger bowls and motors. Also, the only NSF approved KA mixer is a lift bowl model (albeit "only" 5 qt).
The hinge in the tilt head's is probably their weakest point, structurally. There's always a little play in them, I think, at least from toying with a few on store shelves. Probably nothing to worry about for most home use, but if one were to be making lots of bread dough, a lift bowl would probably be better.
The biggest lift bowl I've seen is 600 watts. Not sure about the tilt-heads, but I recall seeing one at 350 watts, which is a lot for that design.
I haven't had too much problem adding ingredients to my 6 qt Epicurean. I don't use it all that much, but made some batches of brownies with it a couple of months ago. It has a pour shield but I haven't used it. I suppose it might come in handy for dusty things like bread dough.