My family came to this country in the late 1600's and settled in North Carolina, moving to Tennessee in the early 1800's......I am the first one to be born outside of a Southern state.....I have lived in Tennessee, still have family there, and have nearly 400 years of people being buried there, homes made, crops raised, lives lived.....I not only "feel" an attachment, I AM a link to those people.....As such, I will defend my home and fight with a Tennessee unit.....
If I lived in Texas at the time, then I'd probably end up fighting for Texas. As for the unit, maybe Co. A "Marion Rifles" of the 1st Texas Infantry, but I probably wouldn't make it home alive in that case. Easiest would be the 1st Texas Heavy Artillery, guarding the Galveston coast.
If I'd been born here in central Missouri in the 1840s, I'd probably try to be loyal to my state, but it's hard to say who I'd fight. I'd have probably been pretty hostile to whichever group was trying to intimidate me at any given time. I would have had lots of options: Out of state Yankee troops, MO State Militia, Enrolled Militia, Southern bushwhackers, Northern bushwhackers, Home Guards, State Guards, Jayhawkers, Redlegs......... I would NOT have been permitted to remain neutral.
Missouri would be the only choice for me. I think the early events that happened in Missouri would have encouraged me to join the nearest State Guard unit, which ironically would have put me in opposing sides with the rest of my (known) relatives in Missouri who joined the Union side up to a year later.
After reading Patrick's post, it brings up another good point - Which side I had chosen would have also depended on which group acted in hostility against my family and I first.