I also replaced my tub w/a walk in shower when it became too difficult to lift my legs over the side of a tub. It has a built in seat in one end & my grandson put up grab bars on the walls.
I had considered those walk in tubs, but thought about having to sit in it till the water drains & also they looked pretty small (I love the "space" mine has). BUT they are MUCH MORE EXPENSIVE than the walk in shower w/seat.
Good luck in your decision.