I'm in the steam camp as well. Diesels are cheaper than steam in almost all models for sale with few exceptions. Overall Lionel catalogs generally have more diesels in them, don't know what ratio that would be from catalog to catalog, but it is a fact.
A few years ago, the second catalog Lionel releases had two or three types of steam locomotives, offered in a few different road names. This was the sum of six pages. The diesels had about 5 different types(maybe more) with quite a few different road names. Granted this is the smaller catalog, and there should be less to make, and sometimes it has been about equal.
The train store I go to has about equal steam to diesel on the shelf, but I'd eager just a bit more of steam(scale stuff). If you factored in the semi scale, diesels take the lion's share easily as there is just so much more. Whether it is command, post war, there's just more diesels out there overall.
As far as producing them, I would imagine that they would be slightly easier overall. Guessing that because less smoke units, plastic bodies for most, that sort of stuff. Besides, you could pick up an AA and a B unit for the cost of just one steam engine(in some cases).
Me, I'm still going to buy steam, because there are only a few diesel engines that I like to look at(older buggers that came in against steam).