I'm working with older games (Duotron and Domino) and have opted for LED in the head and under the playfield, and incandescent above deck. I just like the way they look; I've sought games I might have encountered as a kid in what is clearly a nostalgia buzz, and LED lamps above deck takes away a bit of the magic.
I imagine the more modern games which have a ton of bulbs under the playfield ratchet up the heat AND the cost of transitioning to relamp. I think I've got a lifetime supply of both incandescent and LED lamps hanging around.
One benefit was using red LEDs under the special and bonus inserts - Duotron's were pretty faded (much more so that the green inserts on the game) so with incandescent or LED bulbs, those lamps were more pink than red. The red LED bulbs really made the red inserts really pop without having to replace or augment the insert.