I've never flown Iberia but have used KLM many times from the UK via AMS to travel to the USA and Canada, and even further into Europe in the days before locos began flying direct.
Haven't used KLM for two or three years though so any comments I make have to be taken in that context.
KLM used to be an excellent airline and their fares competitive. However, since around the beginning of the current century I began to notice a levelling down, mainly in the standard of attention and catering in the passenger cabin.
I found the comfort quite good but can't comment on the inflight entertainment because I never watch it. I either read or look out the window.
Prior to their partnership (read takeover) with Air France, KLM's main partner was the US airline Northwest and when a codeshare meant travel was on a NW aircraft I always cringed. They were dreadful: uncaring and rude cabin staff; poor food and older aircraft, at least on the transatlantic routes I flew.
Finally, as Aviador, has touched on, KLM, or rather Amsterdam Airport, can be a black hole for baggage.
On three separate occasions my wife and I arrived home in the UK without our hold baggage. Fortunately, it eventually turned up on each occasion and once was the fault of the check-in clerk at Phoenix, Arizona who incorrectly entered BRU instead of BRS as final destination on our luggage labels. I only noticed it when the luggage was delivered to our home and anyway most passengers wouldn't have a clue about IATA and ICAO codes.
I'm sorry I can't be more helpful than this. However, if I wanted to go long haul again I would certainly do so from my local airport and if this meant using a continental hub such as AMS with KLM I would do so.
My flights with KLM were all leisure flights and always in economy.
