Only you can decide that, of course.
I got the newest version of Thoth just the other day to try, also got Unison. My problem now is that I like both of them for different reasons. So I bought both! 🙂
If you really liked MTNW, then moving to Thoth is the simplest move you can make as most everything about it is very similar in operation. Setting it up is a little more convoluted than MTNW, but not that daunting.
Unison is more OSXified and is a nice multi-pane program. It is quite powerful and works very well.
Frankly for text, both are great. For binary pictures I think that Unison might be a bit better, but I don't do binary usenet.
BTW, both have free trials. 30 days for Thoth from the developer, and a week (I think) for Unison. Get the free trials by doing a Google search.
Hope this helps.