I've not been to Bochnia, but I can certainly recommend Wieliczka. It's very easy to get to (about half an hour on the train from Kraków Główny), and they offer two tours - the 'tourist' and the 'miners' - if you have the time, do both as they are very different experiences, and you get a discount on the tickets for the second tour if you can show tickets from the first.
The 'tourist' tour is the easy tourist one (as you might expect), with all the decorations, underground lakes and churches, statues etc. At the end of it there is an optional tour of the underground museum which is included in the price - I'd recommend this, when we did it we were the only people on our tour who did, and we learnt a lot more about the mining as we effectively had our own personal guide!
The 'miners' tour is a more immersive experience, with overalls and helmets, lit only by your headlamps, giving you much more of a feel for what conditions would have been like for the miners.