PADI you'd enter as BSAC Ocean Diver (with 1 or 2 lectures on tables and about bsac i think).
The next BSAC grade (Sport Diver) is roughly equivalent to PADI rescue diver.
As for pros/cons - it'll be a hornets nest.
If you want to dive in the UK and have a BSAC club nearby then you wouldnt be short of buddies and training would be included in the membership fees.
If you dont/want to dive abroad/dont want to join a club then its PADI
Im not saying more than that because i dont want to start a mammouth flame war
