Mrs IB transited Luton last weekend and that is where her vote goes.
Awful, bad design, total lack of seating at gates, snail paced security.
As with MAN, it depends which terminal you use. I find T3 is slow for baggage reclaim, but otherwise OK, whilst T1 is abysmal for many reasons, both arriving and departing.
As an experience though, LGW (N) is the pits.....and I've given up using Heathrow.