I haven't used the Milt Sparks. I carry a USP45c concealed in a Comp-Tac MTAC. I have a Crossbreed for my LCP so I'm familiar with the differences. I just got a Comp-TAC Infidel for my P2000SK which I've used with both the standard clip and the tuckable clip. I have a USP45: haven't carried it, but am very familiar with the size vs. the other HKs. The USP45 should be workable under a jacket with either MTAC or Crossbreed, but grip length would make it tough to conceal under a tucked shirt with any holster.
The crossbreed will probably be the thinnest profile of all those listed - they use a minimal, thin kydex half-shell and relatively thin leather backing. The Infidel conceals well with the tuckable clip (whether you tuck it or not), but will be less stable this way with a large gun like the USP. If I were going to carry that gun IWB, I'd use the MTAC, as it's the most comfortable (for me) and still very low profile, and I like the clips and the way it "sits".
All those things said... I doubt I'll ever carry a USP45. The USP45c and P2000SK .40 do all I want in a CCW.