I'd go with a shower without any lip to step over instead. My father installed a walk-in tub for my step-mother and she likes it, but you have to sit inside it and wait while it fills up. Then it cools down before very long and you either add more hot water or get out. When we built our home recently, we installed a shower without any lip at the bottom to step over. The floor is cut down for drainage (and reinforced). With this, you can get in and out when you want. You can put a chair/bench/stool in there if you want to sit, and it may very well be less expensive than a walk-in tub.
