I did MRU NBO not so long ago in C. I don't remember any blocked middle seats on the A319... it was business style seats as far as I recall, two abreast in stead of three abreast in Y. But maybe things have changed since then. It certainly seems that way on the MK website.

The service was great.

As to your value for money question... it is indeed subjective.
The seat was far better (but if it's the blocked middle seat system that will no longer apply) and so was the food and wine. There was no avod (as far as I recall). I don't know about lounges etc in NBO as I didn't return on this route.
As you say it's a four-hour flight... you WILL receive food and drink in Y (free).

Personally I would pay the extra, but then I always try to fly in as much comfort as possible if my bank account allows it.

Difficult to give you a cut-and-dried answer though....

Out of curiosity which airline is MN for your MRU-JNB (I presume) leg?