If you take a given cabinet volume and speaker, the sealed box should sound tighter and faster as you noted. The ported box should go deeper and be a little more efficient. The introduction of a port disturbs the phase and timing characteristics of the sub as a whole, also the speaker cone behaviour becomes unruly below the frequency at which the cabinet resonates (most output at such frequencies is derived from the port). In a badly designed ported box you can also hear wind noises as air turbulance can occur in the port. On the other hand though, getting really deep bass from a small box is very difficult, so sealed box subs tend to be larger. A more powerful drive unit and plate amp would also be needed to achieve the same volume levels. If you value punch, speed, timing and accuracy over depth and volume and can accomodate a larger cabinet and bear the cost of more expensive drive unit and amp I'd say go for the closed box.
It looks like with the BK you're not paying dealer mark ups, marketing budget and massive tooling and production costs. Your money instead is spent on the sort of components that can eek out quality low bass. The Peerless XLS 10 is well liked and it alone retails at around the £90 BK are asking for it.
I don't think you'd do much better unless your wookwork is good and you build one yourself.