If I read the OP's question, it was about boosting a HIGH gain amp.
So, if you are looking for a volume boost, the RC is great.
It can add a little grit as well, should you want that, yet the RC has clean headroom.
I like it on any channel for a transparent boost.
Now, the BB with mid is a different animal.
The BB has a TON of gain on tap and some good tonal shaping options as well.
The question is what you WANT fom your pedal.
A high gain amp probably doesn't need more gain, yet I have used the BB with many different amps and it does sound consistently good for that use.
As mentioned above, when an amp is set with "some" gain added, the BB can take it into intense.
Plus it has the tonal shaping controls that are often nice to have is a boosted tone.
Ideally...try both, otherwise if you prefer the versatility I would go with the BB because you get more tonal shaping options and you don't HAVE to crank the gain, even though that can be fun too.
Good Luck