Performance wise, Blue Thunder and Mummert are very close, with the Mummert intake possibly having the edge.
Blue thunder do make improvements. They got rid of the slot in a more recent version and it sounds like they may add dimples or ridges on the plenum floor, soon.
In this thread, you can see an earlier version with the slot, http://forums.y-blocksforever.com/Topic133096.aspx
Ted wrote an article in the last year or two testing a Blue thunder modified with dimples and without the slot. I can look at my issues to see which but Ted might know right away when he sees this. With that mod, the two intakes are extremely close.
One possible benefit of the Blue thunder is the inclusion of the exhaust passage under the plenum, which may be beneficial to those in cooler climates. That also may contribute to the additional expense - more aluminum and probably more complex tooling.
I do see used Blue Thunders on eBay once in a while, sometimes priced very reasonably.