Uprightness of position is a matter of personal preference. When selecting between the 18" and 21" frames, pay close attention to the length of the top tube, i.e., the seat-to-handlebar distance. I stand 5'8" tall and wear trousers with a 30" inseam, and my road bike size is 55cm. I ride a 19" mountain bike, which is perfect on roads and tame trails, but which would be too tall for me for technical work. I also have a 17" mountain bike, which fits me quite well, although with a fair amount of seat post showing. You might actually be OK with either frame size, or perhaps 21" for your wife and 18" for you.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069