Good question.

The most important things to me are:

Comfort and convenience. That's why I started flying Singapore - most comfortable and best service in my experience. But ANA became more convenient and less expensive so that was a consideration (cheaper flights and easier to change).

Upgrades are also important to me. Since almost all of my flying is last minute, I'm more likely to use miles to upgrade than to cash in for a free flight. Even better if I can get a tier status that gives me some free upgrades without using miles?? Sometimes even Premium Economy is better than Economy for me (laptop power in ANA, for instance).

Schedule is somewhat important, but I haven't noticed much difference for LAX-NRT between ANA, JAL, Singapore. As for LAX - New York, I usually fly United, but it's just worked out that way. No huge preference. Again, upgrades and convenience and service are most important, followed by cost.

And as for credit cards, I will generally not keep a balance - just pay it off each month. (Mostly use a debit card at the moment, so not an issue).

Thanks!