I got a regular baby bath - the swimming hippo one from mothercare, and baby Nancy (4 wks) loves it, theres plenty of room for her to splash about and wave her arms and legs! And because theres so much room for her its easy for us to make sure she's had a good wash all over!
It's only my opinion but i think the tummy tubs look a bit cramped!
I agree with redcrx, we had a baby seat for our little one, much easier to cope and she could happily kick her legs about - without risk of slipping etc.