I've never actually flown BA in long haul economy thanks to this forum in various ways. I've flown Virgin in economy and was fine, I'm sure BA would be similar but of course I'm not going to turn down the opportunity to get in a higher cabin.

Frequent BA sales have put club world within reach when at regular prices I would have been struggling to justify premium economy, I've even booked first in a sale fare, something I never thought I'd do.

Depending on how far in the future the flight is you might find it worth waiting to see if a sale appears. Some sales concentrate on economy, others look at premium economy, business and first.

Safety as others have said you'll be fine on either. Also if it matters to you both are 4 engined aircraft.

The A340 has the better seating layout for a couple travelling in economy. The 747 is better in business and both would be fine if you can get an upgrade to premium economy (world traveller plus).