Student: How does everyone decide what neck to use? How does everyone decide how much to play throughout the whole song? For clarity I am in a complete overdubbing situation (where all musicians record separately) so it is hard for me at times to gauge the complete feel of the song.
Paul: This may seem like a vague answer. I promise you it is not! Studio work varies within each style of song from producer to producer. How well anyone does in the described situation will depend on how deeply you have studied commercial music.
There are thousands of variables to each song. Any choice could be wrong or right or just mediocre depending on the producer's directions to you. It's key to have a "creative conversation" with the employer (artist/producer) because there is no one-size-fits-all answer to this question.
When listening to music, study what the bass and drums are playing. Study the entire song to see which musical harmonic and rhythmic choices were made by the piano, guitar, or whatever instruments were used on any popular song. Dig deep in order to hear how the other instruments respond to the rhythm sections parts.
ALL studio players are expected to know this stuff inside out. A deep understanding of orchestrating steel parts is essential and applies to every genre and style of song. From that perspective you will figure ways to add to a song's groove or emotion.
As far as which neck to choose? Always choose the tuning you know best. Learn the E9th, then the C6th. If you have a universal tuning you will still need to learn how the C6th works, otherwise all universals are just the concept of an E9th with added intervals. The D10 rocks!