Get the thicker firebox. You can always get scrap steel and make your own tuning plates, but once that 1/4" firebox is on there, it isn't an easy or cheap upgrade to a 1/2". Stick with the 24x60, if you go to the smaller cooker you'll always wish you had the extra space the larger one provides.
Team: Smitty's Real Pit BBQ
Superior SS-3 - Courtesy of Gainesville Jaycees and Superior Smokers
2 Custom Built Reverse Flow Stickburners
Weber 22.5" WSM
Weber 26.75" OTG
Weber Blue 2000 Stainless Performer