UPS is definitely more stable. FedEx and UPS basically have the market locked up and there are several barriers for a new entrant into that type of market. Even in United's good years they made about 2 billion dollars for the entire year. UPS is making that now in half a year. They pay difference is temporary for about 1 year. Then you will surpass what you would have been making at UAL year 5-6-7.
I've flown both freight and passengers. Right now I'm flying passengers and long for the days of flying freight. I'm actually getting a little sick and tired of smiling at the customers and saying thank you when they are paying $79 to get from NYC to Florida and I have very little disposable income.
Take the UPS job.
GP