Thanks for all the great info and advice, everyone.
ajoknoblauch, for me, both terminals are convenient, but I didn't realize that Colonia Express went out of La Boca. Good to know.
lucha54, is there a bus to Colonia for the day? That would be one long day, no?
I like a bumpy boat ride, Possum, so that's no problem for me.
I checked out Seacat's online reservation system, and it looks like the lowest fare round trip is around AR$500, and that's for the speed boat (the Colonia Express fare above is for the speed boat also). It's also not as flexible with the days (although it looks like there are more options for time of day); you have to plan more in advance (not a big deal, but just noting it).
I'll probably be going more than once in the next couple months, so I may try the speed boat one time and the ferry on another trip and compare.