Honestly she'll be sitting up on her own in no time, so I wouldn't buy yet another tub. I'm surprised that sling didn't hold out for longer. I had a different kind of tub, but the sling lasted until she could sit up on her own. It might suck now, but it'll probably only suck for a few more weeks. I'd personally stick it out.
I'm sure we could have continued to use the sling longer, but she was starting to look really uncomfortable and squished up!