Higher Cost at Annual:newer ('97-'00) 172R/SP or 1970's 172?

Trying to judge which will be more expensive at annual. A newer 172R or S, or going very used, back to a 172M or N 1974-1979 era. Are there a lot more potential pitfalls and expenses that will continue to add up each year at annual for spending less up front to get an older 172? I assume parts are still fairly easy to find because it is a 172, and still not too expensive? Or now that we're reaching 30+ years old, maybe it's better to spend more money up front and get a newer (used--'97-'00) 172R/S. Anyone that has gone though a few annuals with older or newer ones, it'd be interesting to hear feedback.
thanks.