Well to be honest after doing both attractions multiple times in my time, I would say that neither of them will fill you with great joy, they are mainly overpriced attractions, however if you have never done either before, I would suggest the Dungeons, providing you have not done anything similar too it before, because all the dungeons are basicly the same, and its a lot more fun than the madame tussauds, however the dungeons will only last 90 minutes, whereas Tussuads you can spend all day in (if you really wanted too, not that you would!)
Basicly the tussauds is jam packed full of tourists, its like a standing room only concert, and bodies are going everywhere, so I would recommend the Blackpool Tussuads, however not the London one...So I say do the dungeons, on personal choice, however whenever I go, I don't care for either that much, however I will do one of them, depending on where I am in London at the time!
Also bear in mind though, that the Dungeons you get fastrack/priority entrance as well as Madame tussauds, however it will still probably take an hour to get into the dungeons, but only a few minutes for tussuads! So basicly it all depends on your position and timing!
But as I say, the dungeons it probably a better deal, overall! Hope that helps! Sorry if I am a little confusing in my reasoning!