Welcome to the forum!
Good to see someone else from Dayton on here.
M.T.M. (midrange, tweeter, midrange) center channel speakers can suffer from lobing (the two outer drivers actually interfere with each other) when listening from off axis (not in line with the speaker). This is why an identical speaker to your mains is ideal (more often than not it's totally impractical). Bookshelf speakers have the drivers arranged vertically eliminating the lobing. This is why the recommendation towards a bookshelf speaker is an easy one.
Personally I'd spend my money on the mains first (most important) then surrounds followed by a center and sub(s).
Tell us about your room: size, shape, does it open to other rooms etc.. Also how do you intend to use this system: Music, movies, gaming, and/or T.V.