I totally understand your dilemma. It's a topic that is often discussed here on HTF, but the same economics just aren't there for deep-catalog on Blu-ray.
But seriously, it's a title-by-title situation. Lots of the Fox noir DVDs look really great, so I don't feel the need to upgrade to Blu-ray at a premium (outside of really important titles like Laura or Leave Her to Heaven). Kiss of Death? Man, I dunno. That cover is fucking gorgeous, plus Richard Widmark.... Hmmm. I'm not gonna import this, but I might bite when Twilight Time hits it next year.
I actually have never seen this, but I love Coleen Gray (an actress who always delivered the goods even in B-flicks like "The Leech Woman"), so I will probably jump for the UK Blu-ray.