I`ve got the Autodelta one. It gives a nicer sound, better response at higher revs, presumeably because the original is a little restrictive ? That said, the difference is not huge and some of it will be down to the replacement filter.
There are a few extra noisey moments in the rev range; 2.5K and 4K. These are presumably because the baffles/resonators have been removed. I detect a faint hesuitation at the 4.0 to 4.2K range but i don`t know if that is my older car or a lack of resonator chamber problem (technically it could be)
I got a much better response at high revs when I took of the intake into the airbox itself. That has a restrictive bend in it. The problem was that idling was a little rough and the noise was phenomenal - it sounded like a tractor whenever I put my foot down, but in a good way
The AD one is £140 and I would say that it wasn`t worth it. The BMC one has had rave reviews on this site before so could be the better option.
Anyway, why do you want more bhp? Do you drive at 140mph very often? Its more torque you want IMHO and that won`t necessarily be improved by a better flowing intake.
Personally? Don`t bother with either. Buy a better filter for £30 and fit the GTA intake for about the same (search for the fairly recent thread). I suspect that the GTA intake is much the same as my 'no intake at all' mod but without quite so much noise! You will really notice the better airflow over 5K and the engine pulls much better. I`m swapping to the GTA intake very soon myself.