any difference between the these as far as using them for acoustic panels
No difference acoustically. Any variety and combination of similar density Roxul will produce equal results.
Layering 2 slabs of 2" industrial Roxul at 6# density will produce the same absorption as a 4" slab of Rockboard 60.
I've actually worked with both Rockboard and RHT (industrial thermal version) Roxul. Both work extremely well, and are hard to tell apart. Get whatever is available, and you won't go wrong.
It's worth noting that 4# density performs just as well as 6# if you are going for 4" thickness, at a significantly lower cost. The only reason to go with the higher density stuff (like 8#) is for making frameless panels.
PM me if you would like pricing on Roxul.
-- Mark