I've always rather liked the fastback variety where a double taper stay is mitered to fit a cylindrical seat binder boss. I remember reading or hearing somewhere that Bill Hurlow originated that style while building for Condor. Condor Baraccis had this style. Some Serottas do a variation that has the stay overhanging the boss, with a small fillet radius. I'm not fond of stays that plug into sockets in the seat lug, however. I'd tend to be skeptical of any perceptible difference in ride quality. Side attached stays of any variety suit cantilevers better than do fastbacks-that's the only practical (non-aesthetic) consideration I can think of.
Best Regards,
Wes Gadd