If you are trying to buy using Avios, then it's much more expensive to buy via BA.com but much easier if you need to cancel or change the flights. Iberia's website is atrocious and any changes or refunds usually involve having to call. Their English-speaking staff are few and far between in my experience so unless you speak Spanish, go down the BA route. If it's just a cash purchase, and if you are not likely to want to refund or change anything, then booking via Iberia.com is probably best as you will find it easier to choose your seats.

One other hint. On Iberia, they cannot through-check luggage unless all flights are on the same booking. Even if you are on an Iberia transferring to Iberia, they won't be able to through-check the bags unless both flights are on the same booking.

Iberia does usually have far better reward availability than BA in my opinion, and I think their Business product is superior to BA's Club, especially since all seats have aisle access.