The thing to do is to select that which is most like the subject. It sounds like what you cal a split is the split level which originated out of the basic ranch and a similar period style home. You'll have to adjust down as the split level may have greater functionality of seperating living spaces and may be more desirable with equal size, IMO. A cape cod I would think would not be as similar, perhaps being more of a bungalow style, both of about the same period.
Style of a structure is in the eye of the beholder to an untrained eye, including mine at times as there are technical aspects of design that can overlap, the 50s and 60s styles being mixed. The low pitch of the roof, the overall shape, rectangular or angular styles, off-set door/entry near the front or side facing garage seem typical of the ranch. cape, bungalow, saltbox usually having a hinger pitched roof, center entry and deached garage are from an earlier period.
Use the most similar to the subject first in size, style, location, condition and price. IMO Not to say that initially you pull comps in a price range of the expected subject +/- 10-15%, maybe more and see what you have, then narrow the field. Good luck :)