I did not do any interview prep because I didn't have much time, I was coming from another 121 operation, and I felt fairly confident with my interview skills...I was lucky to get the job despite my imperfect interview.
The only thing I did to prep was have a guy who used to do interviews for another airline ask me questions and give me feedback.
If I had to do it again, I would definitely do any sort of prep with any of these reputable companies. I made a lot of mistakes in my interview that I think would have been flushed out in a prep interview. I was lucky to still get hired.
It is not worth losing a great job because you don't want to spend the money or take the time to do a prep course. Invest in your future!