I suspect you know the answer--buoy turns are faster but potentially wetter. Fiona Wylde's version of a buoy turn is the quickest 180 turn I've ever seen--one stroke, bam, done, but you have to have Fiona's balance to even try it. At the clumsy geezer end of the spectrum I can do buoy turns, but not in the fall, winter, or spring.
You can do 90 degrees with a crossbow sweep that continues from the twisted, blade reversed side, back across the bow to the normal side, using the braking force to start the turn and finishing with a sweep that gets you moving again. Any cross-bow turn relies on braking force to start the turn. But if you try to extend that into 180 with a second cross-bow move you aren't moving much at the start, so there's not much force helping the turn, so it's slow.
