I have been in the situation you are describing a few times before.
My advice is to do a good job researching each company before the interview. Google former employee reviews of each company and see what they say. Also search for each companies most recent accidents.
I think younger people (ages 20-35) feel more pressured to accept a position on the spot. As long as you act enthused and gracious for the offer it is perfectly okay to ask the employer for a reasonable amount of time to consider the offer. I think asking for between (3-10) days is acceptable. Here is how you phrase it, "This is a really good offer but I need a few days to think it over. Can I get back to you by next Thursday?
95% of employers are going to say yes to that or at least give you some consideration period and the 5% that would say no, you would not want to work for anyway.
If you feel that it is the better company then there is no harm in accepting it on the spot. Just remember that when you accept an offer you are boxing yourself into a corner a little bit.
If you take time to consider and go to the second interview, then during the middle of the second interview you want to make a smooth segue into the offer/ interview that the first company made you. Saying something like "I just got that same question 4 days ago." I used to be an oilfield recruiter and if I found out that someone was in demand it always made me want to hire them/ increase the pay more.
Like one guy already said, do not second guess yourself and feel like you made the wrong decision. One thing about aviation that you will find out is that the grass is always greener somewhere else. Part of being a confident person is having the foresight to see something 5 years ahead, instead of squabbling over a $5k difference in your first years salary.