The IB5 has got slightly larger gears than the BC, and has a better bearing set-up.
However, I have found that the gears in the IB5 are more fragile than the BC ones
I will stress that these are not scientific findings, but are based on anecdotal evidence, i.e. experience.
Of course, we can reduce likelihood of stress-fracture by shot-peen & polish of the gears, and we have done this for clients who have had specific mounting requirements, that ruled out the use of our BC boxes.
Quaife also list a gearset for the IB5, but, like their BC kit, has ratios more suited to atmo/rally use rather than turbo/road use.