With 6 foot tanks, people usually use a 48 and 24 inch, two 36" or try and find a 60". While T5 are great, they are not really known for filling in 12 inches on each side over a 6 foot tank.
Black Friday sales are when people usually get their T5 bulbs. If you are flexible on colors and stuff, you can often find people closing out KZ, AquaMedic, Giesemann, etc. bulbs for $10-15 bucks. These are all good bulbs.
The cheap eBay chinese T5 bulbs are low output. They can be fine if you put a bunch of them over a tank with easy corals, but they are better for supplement or non-demanding tanks. They are cheap, but not always worth the savings. They work great over fuges to grow chaeto and stuff, but so do $3 GE bulbs, reef bulbs over 2 years old, etc.
6500k and 10k spectrum can add light and colors that other bulbs and LEDs do not have. True Actinic can do the same. It can take a while for the corals to respond, but there are real differences and growth. There is a reason why coral farmers still use 6500k bulbs over their corals. You can find the masses or posts online about how corals from these folks lose some of their color over time in tanks with limited spectrum - Battlecorals is one of them where heavy-blue lit tanks without true UV (not lying LED manufacturer UV that is just violet) or IR lose the high contrast and even some of the color.
