Before repatriating to NZ from the UK late last year, I made the trip to MK to visit both BP and TNMOC. I was well aware that they were separate organisations before I set out, so there were no surprises on that score.
For a geek like me, TNMOC was awesome. Hands on, all the classic computing hardware from my youth, and cool flashing lights (I loved the Decatron). I saw Colossus and Tunny. I had a happy morning wandering, playing and chatting to the volunteers (and even got to help boot a classic Unix CAD system). It was really cool, and worth the trip on it's own. However, for all the love and technical skill on display, it was obvious to me that TNMOC needs more investment and space.
Then I headed to Bletchley Park - more expensive, but more going on. I knew many of the details, so I decided to forgo a tour, and just wandered round by myself. There was lots to see, but the displays were a bit busy, and the information panels were small and a bit wordy. It was less interactive, and I ended up considering it pleasant for a visit but not enthralling. It also needs investment, but it also needs better management and organisation. Sadly, from the recent commentary, it may not get either.
However, I do suggest doing both. The Bletchley Park ticket did allow a return visit, and I am sure the guided tours add more value. And the grounds are nice if the weather is good, and a picnic/walk would be a great use of time. The current management may be prats, but there is a story being told at the park that should be experienced. But TNMOC is just great.