My tuppence from our experience:
DS with 10 GCSEs but an attitude problem- started 4 AS levels; Maths, Physics, Geog, Economics. The Maths was a close call has he's just missed an A in his GCSE.
Basically, AS's just weren't working for DS, as was apparent by Easter in Y12, and given that I don't believe a couple of mid to low grade A levels are worth the paper they're written on, we investigated his options and hit upon a BTEC, 3 'A' level equivalent in Computing, restarting Y12.
DS duly got for his AS levels, a D in Economics, E in Geog, U (!) in Maths (having stopped attending lessons) and had dropped Physics altogether.
Initially he was going to start Maths again with his BTEC but investigation demonstrated that RG unis are pretty much only interested in an A at Maths A level alongside the BTEC to do Computer Science. Subsequently I have had a second DS doing the 'new' style GCSEs and the local sixth forms require a solid '8' in Maths order to allow Maths A level entry....
I knew DS was unlikely to get that A, so he did just the BTEC.
IMO 3 A levels would have been harder; but the BTEC requires a lot of constant input, no 'last minute revision'; but if a DC isn't capable of 3 'hard' (i.e. not 'soft') A levels and to get solid A/B style grades, they'd be better off doing the BTEC.
DS got Dstar; Dstar; D and is now at a well regarded ex-poly (sorry to be blunt, but not London Met..., but with an 80% 'employment rate') doing a Computing degree (with a £1500 academic bursary...
)
So, imo, if a DC want to do Computer Science/Computing (how they differ depends on individual universities- there is overlap) and is capable of 3 solid, 'hard' A levels and in line for A/Bs, I'd say go for the A levels; if they're more likely to get something more like CCC/BCD, I'd say they'd be better off 'committing' to Computers at uni and to do a BTEC.
A major advantage to DS has been that having studied Computing for 2 solid years (9-3, 3 days a week, no study periods or 'frees'); he's quite a way ahead of the '3 A level' entrants on his course. And- he's that one year older and more mature!
For the record, 4 out of the 8 DC doing DS's BTEC are at uni doing Computing now, the others are all in computing work (another advantage of BTEC!).