i used to do baby backs but have switched over to st louis cut spares. if you can get a deal on the full spare, its easy to trim them to st louis style. the reason i like the st louis style is that they are usually evener (is that a word?) in size from one end of the rack to the other. that makes for easier smoking. the bb's usually have a big fat end and the rack tapers off to smaller and thinner on the other end which gets overdone. while you can get a good even cook on bb's, for me and my wsm, its easier with st louis. plus there is a lot of meat on a st louis style and when done right i think they are tastier with a better texture than bb's. and they are usually cheaper.
