Gentleman Aviator : KLM mainline is a very 'senior' airline.
You start as a 'Co-Co pilot' or cruise pilot on a heavey, then go RH seat short haul, before taking either a shorthaul command or RH seat long haul.
KLM has a 'glut' of pilots at the moment, it is extreemly difficult to get into (not to say BA isn't either, but a bit more practical), and if selected you get put in a hold pool, similar to BA.
KLM's pool has hardly moved in years ...
BA has 1000 pilots to retire over the next 5 years out of 3500 total.
So, with BA carreer progression is a real factor, you could have a LHR shorthaul command in that time, or a regional command within 2-3 years.
Airlines are all about seniority.
Plus as a junior FO in KLM living in NL (paying 60% tax) you will take home a fraction of what you would with BA.
Not to say KLM isn't a good airline, I flew for a subsiduary of them and saw a lot of the KLM operation and crews, great set up and working enviroment .... but no expansion or retirment bulge .... unlike BA.
At the end of the day, long term, you will never go wrong working any flag carier, athough proogression will be slower than a charter or low cost operator your somewhat more secure and the perks (staff travel, pension, loss of licence insurance, family health cover etc ) are all thrown in. PLUS you will not be subject to roster distruption and can plan your life.
With both BA and KLM the unions are extreemly strong ....