My LO is four months, but he's over 16 pounds (getting weighed today - eek!) We have the "Whale of a Tub" that has been working beautifully for baths until now. At this point, he just learned how to stiffen his body to get his bum over the little hill that keeps him sitting up. Plus, he's added a twist, so that if he moves fast enough, he could end up with his face in the water.
At this point, the baby tub doesn't seem to be a great option, but I don't know how to bathe him safely in the big tub either. He's so squirmy I cant imagine holding him with one hand and washing with the other. I thought about trying to lay him down with a big towel under his head so water doesn't get in his years.
I'm hoping one of you has some great wisdom on the subject. What are you doing/planning to do? (Ds doesn't look like he'll be sitting up on his own any time soon).