Mountain bikes are a lot more tolerant of size variation. I like 19's for off roading but it's on the ragged edge of being too small for me and I need to show a lot of seat post. But the agility that comes with the smaller frame is well worth it. However it means that when I used my old Trek 970 with the 21 inch frame there was no problems with it since I used it with fenders and a rack for commuting and it never saw dirt while wearing knobbies at all. I'm 6' 1" to fit this size range so we are pretty much on par for sizing.
Riding the 19's in the street makes for a more upright and casual positioning unless you want to force it to a more aggresive posture with the right stem. With a 21 inch it's much more like one of the new style sloped top tube road bike frames but set up for 26 inch wheels. If you want to roadify it then 650's are an option.