A C should not feel tighter than a B, since C cups are larger than B cups.
However, buy whatever works for you. Don't bother with fitters, just try on a few sizes in the store. Bras made by different manufacturers (and in different materials) will fit differently, so you always need to try them on. Once you've got a bra on, put your clothes back on over it, to make sure it works for you in terms of appearance as well as comfort.