I've not flown Iberia premium economy, but I think the long-haul product is likely to be more differentiated (whether for or against BA) than the short-haul.

Unless you strongly prefer dark leather to light-coloured cloth (or vice versa), there is little difference. On the plus side, BA headrests have more support and BA crews are generally more engaging. I prefer the Iberia buy-on-board offering but rarely take it. Iberia can be stingier with hand luggage allowances.