Gels tend to have fewer calories per-serving than bars, so I suspect that's what your friend is getting at. For example, one Hammer gel has 100 calories, whereas a Clif Bar has 250. Additionally, most bars have protein in them, whereas gels do not. Protein isn't so important when you're only riding for a short period of time (nor are carbs, for that matter), but for longer rides it's generally a good idea to include some protein in your food.
I prefer to start with gels (mostly because Hammer espresso gel is delicious and I like the caffeine at 530am), then switch to Clif Bars and Perpetuem after 2 or 3 hours.