I served with 63 in the 90's and they are a top outfit. Back then the physical standard was quite high and you certainly got to get about a lot. They really play up the SF side of the role a lot and if you are looking for the opportunity to some operational tours then this is the way to go. The selection is quite hard and I think out of 15 recruits when I joined only 6 lasted.
Also, it can be a real trial in itself getting into the place because the RA plebs on the gate really give it some attitude.
I had a chat regarding selection for 63 sigs (which i didn't go for in the end but that's besides the point) with one of their officers who reckoned, out of boys who get less than 8.30 on their BPFA, there's a 50/50 chance of passing. In his words, not impossible for a "fit bloke with a bit of grit about him".