Solid where needed, and expanded everywhere else. You'd be surprised how fast the weight will increase by using solid steel for the entire floor. When I built mine, I used solid for the entire base. The only problem was that once I got done, the trailer weighed in at 3900 lbs, but I only had a 3500 lb rated axle. By the time I replaced the axle with a 5200 lb rated axle, which required new tires and wheels, that little mistake cost me $2,200.
CBJ, 15' custom BBQ trailer with a Backwoods Competitor, Pitts & Spitts off-set, & Santa Maria grill. Ole Hickory CTO, 1 BGE, 2 Weber kettles and 1 Smokey Joe, BWS Chubby, Sonoma gasser, and a BBQ store full of everything BBQ.