Well, shoes are pretty personal. There are a handful of brands that make great shoes, and within those, if a given model is Speedplay compatible, it all boils down to which one feels the best on your foot.
Sidi, Diadora, Pearl Izumi, Shimano, Nike, and Carnac all make excellent shoes for recreational and pro use alike. I've got a pair of Pearl Izumi Vapers that work pretty well on my feet, and seem a good match for my Speedplays.
maybe it was just the models i was trying on, but Carnacs seem to be a lot narrower than they used to be. of course, i've got 8EEE feet, but they used to be perfect and now quite tight.
for my Speedplays, i'm using a pair of Northwave Evolutions with the Time sole plate which is perfectly flat so you don't need to use the snap-shim with Speedplays. seems pretty similar in terms of stack height to my old Carnacs, but haven't really measured the sole thickness.
I currently use DMT shoes with my Speedplays, but I used to ride on Time shoes...they were awesome. Too bad I grew out of them. Speedplays worked great with the Time shoes - no adapter plate necessary because the Speedplay cleat is drilled for four hole mounting - just like Time shoes!