When leading, a lot of players always opt for longest/strongest, others always look for the most safe alternative. Some well-known names, Tony Forrester and Peter Fredin comes to mind (among others), would rather drink toilet water than lead away from a K vs a suit contract.
I'm in the mixed zone. The auction influences the choice to a large extent, as it should of course, and I also look for 'middle of the road' vs NT, i.e not most attacking or the safest.
I'm more passive vs suit than NT as long suit tricks come into play in the latter case. By this I mean that even if the lead costs a trick, it may come back later, if you can cash established tricks (with no little trumps out there preventing that ;-)
Freddan had to find a lead vs 3NT from:
The auction went 1S (4+) - 2C (gameforce with bal or clubs) - 2NT - 3NT. What would you lead?
Here the choice comes down to a heart or a diamond. Frederic started with the 7 of D (not risking a beer on that one ;) playing Schneider-Rusinow, a choice I like without any useful heart spots. That was a real killer on this layout:
South somehow managed to open with 1S instead of 1H. After the diamond lead, we beat it which only happened at a couple tables in the rather large field (50+ tables).
Too many players are addicted to aggressive leads. A lesser number are addicted to passive leads. Stay away from addiction and when in doubt: go passive.