Tubes for frequently-used colors, pans for select colors.
As far as quality goes, there is no difference between tubes and pans. A pan is just tube paint that has been poured into a little plastic mold and allowed to dry.
Tubes are usually cheaper than pans in the long run. You can buy empty pans and refill them several times from a single tube. I use tubes for the colors that I use every day.
For select, infrequently used artist grade colors, look for individual half pans. It’s cheaper to buy a small amount than an entire tube that may be used rarely. Toss them unto an empty Altoid mint tin, and you’re ready to paint!