If you're used to the bars, then I would stick with the bars but perhaps supplement with a gel every so often. At a slower pace, solid food is probably the best choice but once the pace gets more aerobic and sustained solid food takes more time and energy to process. So for me, on a hilly metric, I use an energy drink and gels along with electrolyte caps. It's easier on the stomach and the results are more immediate. Test what works best for you.