Both are popular aging-in-place upgrades, but they solve the problem differently and have different tradeoffs.
Walk-in tubs
A door on the side lets you step in without lifting your leg over a high wall, but you must remain in the tub while it fills and drains, which some find impractical.
Curbless (zero-threshold) showers
No barrier to step over at all, generally considered the gold standard for accessibility, and typically better for resale value than a walk-in tub.
If budget only allows one, a curbless shower conversion generally offers better long-term flexibility and resale appeal.