everyone is gonna tell you to get the q over here cuz its a good sq sub that gets loud.
i say get the bl cuz its more efficient and you can get a flat wind which will drop the fs a bit, increase power handling and increase efficiency also the hyperpole will help the power handling as well. theres other options to further increase power handling but i believe they drop the efficiency.
The Q in the proper enclosure, with a 1000w amp = awesomeness.
For the enclosure, sealed or low tuned ported. I run two 10" Qs sealed myself and it is the best sounding system I've ever had. Output won't win SPL comps but its plenty loud enough for a daily driver.
What kind of music do you listen to? what amp do you have? do you plan on getting a proper custom box, or using a prefab? I wouln't get the BL if you plan on going sealed, just throwing that out there (never had a q or a bl though)