Look at it from a practical standpoint ... if he's going to a city prison he wont find any gear there, so he basically has to have it with him or have somebody bring it in.
With a 50-100mg/day dose of anadrol we're talking about 30-60 tabs for the 30 days.
With 45mg/day of dbol we're talking about 270 tabs.
For that reason I'd pick the drol because its less stuff to carry. Furthermore you would have to take the dbol more frequentely which increases the risk of getting caught.
As for effectiveness, from experience dbol is more effective and safer. But for 30 days I doubt that there would be any significant differences between both. Especially considering that it would mostly be used as an antocatabolic agent: to prevent muscle loss while locked it. Considering that the food supply will be lower than what he is used too and that protein will be very low, all he an hope for is maintenance.