I prefer warm, though bright has it's place. For warm is great for recording; especially to digital. Bright I'd use for highlight parts or when stacked against other instruments in mix. Live, I still go for warm first.
For playing by myself or if I were fortunate enough to be playing a coffee house, open mic night, or other similar situation, I'd grab a warm sounding guitar. In fact my go to acoustic is a jumbo, and really mellow.

If I were in the context of a country band with mandolins, maybe a steel guitar, a violin, I'd probably grab a bright guitar, if only to compete for my spot in the mix.

When you're dealing with AE guitars, you can modify the guitar's spectrum quite with the provided onboard EQ, or trust your sound guy's ear, to position your playing for the best possible effect

There are other things to consider in a guitar's sound beyond the "black &white" comparison of "warm or mellow".

Solid or laminate, type of woods used, personal taste, how well any guitar fits with your vocals, even string alloy and gauge are really important in contributing to your musical identity.