I would do a flight at the first possible chance, so you have time to reschedule in case it gets canceled (it is often canceled due to weather).
I used Talkeetna Air Taxi 2 years ago due to a coupon. All three, plus a fourth, are very reputable companies.
You need to see if the flights are a summit flight (i.e. flies over the summit of Mt. McKinley and requires oxygen), a "summit view" flight (you may or may not see the summit depending on weather) or a flight with a glacier landing. All have their fans.
I did the glacier landing. If I did it again, I would probably do the summit flight, unless I had a deeply discounted coupon for another option.