What variables should I consider when selecting a key?

I wonder oh I wonder... do some composers refer to a synesthesia that coheres to a key due to its symbol's (being a letter of the alphabet) representation? Such as color, shade or "hidden meaning"... I feel that many musicians fall under a similar category, depending on the instrument being played. Thus any internal visual is linked to a feeling/idea, and that feeling/idea is linked to a sound, making a rare connection between inward feeling and inward sound, both of atypical proportions, expressed through music, which is a type of synesthesia. Much like nonsense. Infact, music is not far from how nonsense is expressed. But one must interpret sense to determine nonsense. A musical key would be too universal of an interpretation to limit one's ideal of sense.

Any person coheres certain keys to certain instruments. But I do know of a brilliant piano composer who entwines his music and synesthesia. (along with others of this abnormality)

You should go with whichever key fits your mood...whichever one you like and feel that notably delivers the composition to its best. And your opinion is the one that should matter most! Coming from a composer's perspective, I feel happy when my key is appreciated by others, but I feel happier being able to chose whichever I want. And I want what I want, not what other's want... but getting some ideas online is a good start. If you're clueless, look for more defined opinions such as this one.

Music is subjective, and any objectivity in music is merely a form of subjectivity being disclosed.