I've been to Colonia three time since September and have used the Buquebus ferry for all three trips. They have a website that sells tickets in ARS (
www.buquebus.com), and a website that sells tickets in USD (intl.buquebus.com). They have a Uruguayan website as well. The USD website is sort of a ripoff. For example, a first class ticket to Colonia two weeks from today is $72 on the USD website or 60.860 on the ARS website (which is $54 USD with Visa's current exchange rate).
Buquebus has three classes: tourist, business, and primera (first). Until recently tourist and business were the only options to Colonia, primera was just added recently.
Departing from Buenos Aires, business or primera will allow you to sit in a private lounge while waiting to depart, you
should be first on board the ferry, and you
will be first to disembark when the ferry arrives in Colonia. Disembarking the ferry first isn't really a big deal in Colonia because you process through Uruguay immigration before you leave Buenos Aires, so it really doesn't matter. In addition, business and primera have their own seating cabins with more space and more comfortable seating (and a lot quieter), and you should get a glass of champagne as well.
Returning to Buenos Aires, boarding in Colonia is a free for all and there's no lounge for business/primera customers, so there's no real pre-boarding benefit. It's still nice though to be able to watch everyone else doing a mad rush to get good tourist class seats, while you can board dead last knowing that you have a separate cabin awaiting you.
During my first two trips, the main benefit of business or primera when returning to BA was disembarking first to be first in line for Argentina immigration once you dock in Buenos Aires. As a tourist class passenger I could see myself easily standing in line at immigration for an hour, whereas business/primera got me to the front of the line and through immigration and the customs X-rays in five minutes or less.
However, this "benefit" may be moot now, because during my most recent trip to Colonia two weeks ago, Argentina immigration was now taking place in Colonia for the first time. If that continues to be the case, then there's really no need to be first in line to disembark in Buenos Aires. You might get through the X-rays machines a little quicker, but that's about it. I'm not sure if Argentine immigration now being stationed in Colonia is a permanent thing or just for the summer.
So, as it stands now and unless anything changes, business and primera gets you a departure lounge in BA, a quieter cabin on board with slightly better seating, and a glass of champagne. You'll still be first to disembark in both directions, but the customs and immigration fast-track benefits are pretty much negated now that Uruguay and Argentina immigration are occuring before departure in both directions. I'd still pay the extra for business if it were me because I enjoy the quieter cabin, but if that doesn't matter to you then there's no other downside to sticking with tourist class in both directions.
Hope this helps!