Sump Pump Choices?

I think if it were my place I'd go with a 1/2 hp. submersible unit, and don't buy one of the cheap $79 units Home Depot carries if you want it to last longer than a couple years. The bearings in submersibles are sealed, watertight, as you might expect they would have to be, so the clay you mention shouldn't be a problem, other than the possibilty of it building up on the bottom of the pit. The 1/2 hp. should be strong enough to restart regardless of how much water is in the pipes.