I bought a genuine Bruce 7.5kg for my RIB - chuffed when I found it on Ebay. It turned out to be too big to fit in my well and would be a pain to get through the hatch in a panic.
I have a 5kg Bruce as a working anchor and it is too small for an emergency.
Reading the tests etc the perfect anchor seems to be the Delta. It is basically the old CQR but with a solid shank. Unfortunately the 2 sizes they do suitable for us are 6kg and 10kg. Quite a gap in between but from the tests the 6kg easily outperforms the 7.5kg Bruce. The Danforth works very well in mud but less so in sand and is useless in rock.
The Delta is quite expensive at £84 but they now sell a copy for £22 - it is identical but the finish isn't quite as good.
Mine should arrive tomorrow.