Don't quote me on this, but I *think* the difference lies in the type of rock that's used. Gravel is well....gravel. EcoComplete on the other hand is like lava rock or something similar. It's super porous and is able "hold on" to nutrients dosed into the water column, thus keeping them around longer for better usage by the plants (referred to as "CEC;" cation exchange capacity). It's also supposed to come in a water-based solution that contains bacteria, but I'm not sure how true or helpful that is.
For pros and cons, there aren't really too many pros to gravel when compared to a 'premium' substrate like EC. Gravel would be easier for plants to root in since it's more coarse, but that's about it.
That said, both substrates are inert so no matter which route you went, you'd have to use root tabs if you had heavy root feeders like swords. I personally chose Floramax over Eco-Complete. It's the exact same product (a little coarser, though) but without the bacteria solution and, when I bought it at Petsmart, was almost 12 dollars cheaper. :p
The only thing about Floramax is that people have said it clouds the water initially. It did, but a few 50% water changes and the bio-magnet clarifier they give you in the bag takes care of that.