The hit you will take on early withdrawal of your retirement will be worse than the one you would take for a student loan. Granted 30ish years of airline pay seems like it would make that irrelevant, consider the unforeseen possibilities that could cause this not to pan out.
I vote student loan (interest and payments are very low while you are actively enrolled), supplement with any scholarships/grants that you can get. Also look for schools that reduce your R-ATP minimum total time to 1000 hrs.
Some community colleges even offer a 2 yr associates with flight time, that rolls right into a state schools program for the last 2 years, for the budget minded.