Technically, RTV sealant was a factory money saving scheme. Less gaskets to make and stock.
Technically, you can use either with similar results. The problem with the RTV is that if both mating surfaces aren't perfectly clean and free of contaminants and oil, it will leak. A gasket will pretty much seal once Titan'd. Please note that even though some gaskets were used in various places, there are times when RTV is used in conjunction with gaskets, like at the corners or in difficult to seal areas. I doubt that you will run into this with the oilpan, but check the gasket for voids or open areas that may need RTV. Sometimes a bit of RTV is also used to hold a gasket in place so it won't move while installing the parts.
Basically, you can go either way. The gasket is a more dependable method, at least on a replacement due to the possibility of oil residue remaining on the parts.