If I had to pick it I'd always put LEDs in the GI first.
For Loco it was out of curiosity I did the slings then I just did it all.
For months I thought these games suit incandescents but I've found once you've upgraded you realise how much incandescents suck the life out of the artwork . Especially those blues and whites on the slingshots and lane guides. A warm or sunlight comet brings it to life.
Led in GI brightens up a game and makes things easier to see. I don't find the change with inserts as big, they'll pop more but you lose the fading effect when inserts flash which I like.
All my Bally Williams games have LEDs throughout but with ledocd or afterglow boards so you retain the fading effects.
That said, I am tempted to put a few LEDs in select inserts just to make them more consistent with others. A few just seem dim, I think because the inserts are damaged underneath from years of incandescents.
One minus for LED GI is that flashing at the end of game time bonus, In a dark room it's blinding!