i think the biggest difference, IIRC, is that some of the top of the line ones (is it the DLX?) have "memory foam." However, that said, I find ALL the EFTAs to be quite good in the padding department. but i think some people have said they think the padding is better on the top model--i just can't really tell.
also, the same top of the line one has the fancy Britax-style alligator LATCH clips, instead of the basic hook ones. a nice feature, if you are really into that.
i think those are the 2 big differences. it's never been enough for me to spring for the top model--the base model has been great for us.
BTW: I'm due 7/7/09--same time frame!