I have not flown KLM long haul in Y in a few years but from memory Economy Comfort (flew JNB to AMS) was a bit like AA Main Cabin Extra- same seats and same service, just a bit more legroom.

I cannot comment at all on LH but from past experience KL and BA long haul Y really is much of a muchness. If the difference is £100-£200 then I would go with BA simply on the basis that any cost saving is not worth the hassle of the increased risk of losing baggage, missing connections etc.

However if the saving is greater, especially if taking wags/children, then it might be worth doing, particularly as AMS would normally be an easy transit- though I should add that with current refurb work it might be different.