As a unaligned player in a heavily skewed area, what faction should I choose?

I would chose based on who you want to play with. Ingress is a cooperative game and physically going around with a group of friends had huge advantages. No matter what the current state of your area is, if you have friends you want to play with, the best thing to do is be on the same faction.

Playing the underdog is more challenging and sometimes more exciting, but it can also be discouraging if the balance is to far off for too long.

Playing the dominant faction gives you lots more opportunities to vary you're plays. Alone or with friends, stocking up and making raids on other cities, etc.

Both ways have advantage, and how you level up is going to be quite different (although possible both ways).

Always chose the weaker faction. Maintaining portals gives you much less than hacking and taking over portals. So you need to hack portals to level up, and if your faction owns all the portals, you have no easy way of getting to the top, and the game will become boring for you quickly.

Playing in an area with lots of friendly portals will allow you to collect a lot of inventory (hacking friendly portals gives you more items than enemy), but levelling up will be slow (especially if you don't go 'raiding' nearby areas on weekends). Playing in a mostly hostile area will provide you lots of opportunities to gain AP and level up, but you'll likely be short on inventory most of the time. However other players can drop items for you.

Currently I play in an area that is 90%+ enemy, and I don't have the gear to break most of their portals. But some friendly players drop caches of items for me from time to time and with those I can break some of the lower level portals and claim them for myself for a short time. It's hard work but I'm making progress.

I'd recommend joining the weaker faction and getting on the Comm to find friendly players nearby: some people take this game pretty seriously and will travel quite a distance to help you.

Being on the weaker side also gives you a great incentive to recruit, I persuaded someone near me to join up as well and together I reckon we can actually get a field up at last :)