It is totally fine if you want sand or gravel. I am going to throw in my opinions and experience here though to give you a bit of different perspective.
I have kept cories for years on both sand and gravel. I have never noticed any different in their behavior at all between the 2 substrates. I fully believe that warnings about gravel wearing down cory barbels to be a myth. Poor water quality and bacteria build build up will do this, but a substrate has to be crazy sharp for that to happen. The regular aquarium pea gravel does not fall under this category, I am talking pure glass edged gravel. That said, I know of several fluke type situations where people had cories lose barbels on super sharp edged sand. So, the main point is just to use a little common sense about it, and keep the substrate clean. Gravel has taken the rap for a lot of people's poor tank management over the years. Though, I am sure there are probably several fluke cases where some person found crazy sharp gravel and then this happened.
Personally, I don't like my one sand tank left. It looks cool, but I find it far more annoying to clean and move things around in and I won't likely set up another one anytime soon. That is just preference though. Many people like it a lot better.
I agree. I have a mix of sand and gravel in my tank, and I've not noticed that the cories prefer either area. They go where the food is! 
