Dive-Computers.net

Short answer is, it makes no real difference.

So long as you have a valid diving certification issued by a recognised training agency (of which PADI and BSAC are but two of many), it will be acceptable to any dive centre in the world worth diving with. You might be asked to do a ‘check-dive’ or ‘refresher program’ if it’s been more than 6-12 months since your last (logged) dive, but this would be the case regardless of which agency issued your certification.

The BSAC Ocean Diver and the PADI Open Water Diver course contents are broadly similar in terms of both theoretical knowledge and practical dive skills (since the laws of diving physics apply to everyone), and hence require similar classroom and in-water tuition time (including 4-5 qualifying dives). As such, both courses will qualify you to plan dives within roughly similar limits (i.e. diving within no-stop time limits, to a max. depth of 18 [PADI] to 20 [BSAC] metres [60-66 feet], at familiar sites, with a similarly- or better-qualified buddy).

The only real difference between the courses will be the dive tables on which you learn to plan your dives, either the BSAC ’88 tables or the PADI Recreational Dive Planner (RDP). The BSAC ’88 tables are more comprehensive (e.g. including detailed calculations for ascending to altitude), and also give limits and ’tissue codes’ for dives with decompression stops. The RDP is less complex, and provides greater flexibility for no-stop divers (especially if The Wheel version is used), but gives no information for planned decompression stops.

Both BSAC and PADI (and I imagine most other agencies as well) allow several different methods of structuring their entry-level courses. BSAC used to be (in)famous for their more ‘club-oriented’ approach, where students would do the theory and skills training e.g. one night per week for a month or two, followed by a weekend to do the qualifying dives, as opposed to the ‘intensive’ [i.e. resort-style] course (4-5 full days) under which many PADI divers qualify.

However, these days the difference in approach is not so agency-specific. BSAC courses can also be completed on an intensive schedule, and the PADI OWD course can be spread out over several weeks/months. It’s basically down to the dive centre where you train to decide how they structure their courses (although the better dive centres will take your scheduling needs into account).

At the end of the day, the biggest influence on your diving ability at the end of the course (apart from your own willingness to learn) will be your instructor’s teaching ability and attitude, rather than the badge on your card. I would recommend you visit both shops, ask to speak with their instructors, and sign up with whoever makes you feel more comfortable/ excited about learning to dive. The price of the course should be very much a minor consideration–and bear in mind that the ‘cheaper’ course may have ‘hidden’ costs (e.g. you might be expected to purchase some/all of your own gear prior to starting the course).

Pretty well covered in the other answer but there’s one thing I’d like to point out. BSAC is , for the most part ( with a very few exceptions) only offered in the UK. Diving conditions are challenging in UK waters. The PADI instructors there will likely teach above and beyond the PADI norm elsewhere. The BSAC instructors on the other hand will teach a course originally designed to take those challenging conditions into account.
In a way…the difference is a bit of a badge of honor. A BSAC diver I will always consider a slightly better trained diver. I know for certain that they have been trained for adverse conditions. A PADI diver may or may not be, depending where they took that PADI course. That BSAC cert won’t hinder you. If anything it will be a bonus when diving elsewhere.
That’s coming from a Canadian , cold water PADI diver.