Both are used frequently and I don't think one is better than the other. Start out with the one you are most familar and comfortable cooking. If after several contests, you are not pleased with the results, then make the switch and compare.
I've used both with similar results. Last season I went exclusively with St. Louis spares mainly because they were cheaper.
Lonestar 24" Vertical offset w/ Insulated Firebox & Performer Platinum
Former owner: Jambo Backyard, Klose BYC, Med. Spicewine, Pitts & Spitts, XL & (2) Med. BGE's, 22" WSM & (2) 18" WSM's, 18" & 22" Weber Kettles