I flew a contract that required both (Western US). We sat on stand-by with the tank attached and would launch on initial attacks with it. There weren't many natural water sources around so we would rendezvous with a water-tender and ground fill with a hose. The tank worked well for the PJ or grass fires in that area. It also had a really good foam delivery system. To use a Bambi bucket in that situation we would have had to have waited for the water tender to fill a pumpkin first and then dipped from it, delaying us by 20-30 minutes. Here the tank was great.
When we got up into the taller fuels, I preferred to switch to the bucket on a 50' or 100' line. You didn't have to, it just felt better for me. The Bucket allows you to steer the drop without having to steer the entire aircraft. The Bambi bucket was much better for spot drops too. Doing a spot drop with the tank was tricky. If you got low enough to be effective, you stirred up the fire with your downwash. If you got high enough to dissipate the downwash, the drop would atomize into spray and become ineffective.
I'm guessing that two pilots, one flying a tank and the other a bucket and each equally proficient in his own could do an equal job. The above is just my own opinion.
However, given the choice, I would take the Bambi Bucket. If not for any other reason, then because it was more fun.