A side has one shoulder so yea its equivalent i guess the butt is thicker if I remember right the belly is loose, stretchy and thin. Most folks buy shoulders because they don't want to pay for belly leather.
You can use either, the side will have some belly that is not good for strength but you can use it for linings and such. sometimes the shoulders have more fat lines. I don't think a double shoulder would build all of those and a side would be iffy too. my recommendation is to buy a side that way you can learn about the whole thing at once and how to use it. I feel you will need more than one of either so learn with the side first, the belly isn't a waste it is good practice stuff and for other things as well.
But remember your sheaths , belts and holsters may need to be different thicknesses and that's the next quandary.