Two sports bras in the same size can feel completely different, and most of that difference is in the back. The band does the supporting, but the back shape decides where the load sits and how much you notice it.
Racerback
The straps converge between the shoulder blades, which pulls them inward and stops them sliding off sloped shoulders. It opens up the shoulder joint, so it suits anything with overhead movement. The trade off is that it is harder to get on and off, and it shows under most vest tops.
Ladder back
Strapping split across the back spreads the same load over more points instead of concentrating it on two straps. That means less pressure in any one place through a longer session. It also breathes better, because the gaps in the strapping sit exactly where your back gets warmest.
Longline
A deeper band means the support is shared across more of your ribcage rather than resting on a narrow strip. People who find standard bands dig in usually get on better with a longline. It also stays put when you are moving between floor work and standing, which is why it doubles as a top.
The band matters more than the cup
Most of the support in any sports bra comes from the band, not the straps. If you are adjusting straps tighter to feel held, the band is too loose and you are about to give yourself a sore neck. Size the band first, then judge the back style.
A quick test in the changing room
Put it on and jump five times. Then raise both arms straight overhead. If the band rides up on the jumps, go down a band size. If the straps bite when your arms go up, try a ladder back or a longline before you try a different size.
Washing it so it lasts
Cold wash, no fabric softener, air dry. Softener coats the elastane and that is what kills the stretch. Heat does the rest. Four way stretch knit holds its shape for a long time if it never sees a tumble dryer.