While the Fairgrounds are a much more straightforward method (play, earn money, celebrate) of obtaining treasure, Coli farming is generally faster since you get to do either or all of the following:
1. Train dragons to exalt for profit, or to help your flight during dominance pushes. There are very low chances of getting a few gems along with treasure when exalting. Trained and stoned lvl25s highly recommended for this, but not absolutely necessary.
2. Sell any valuable drops you get from Coli in the AH or forums, like Swipp items, certain familiars, apparel, battle stones, crates, even food, because they pile up. During festival weeks, monsters that correspond with the current festival's flight also drop festival currency, with which you can exchange special festival items that, more often than not, will retire and therefore rise in price over time. You also get a chance of being dropped skin or accent chests, which will also retire after the festival's over. It's better to have some lvl25s for this since you'll get more drops faster.
3. Any other drops that can't be used for Swipp or Baldwin and pretty much have no in-game use can be autosold from your hoard for a bit of extra cash.
tl;dr Yes, please get some lvl25s as soon as you can. It's a good time/money investment.