Here's my $0.02:
I had a similar experience. I'll give you my numbers just for some perspective. I graduated college about 4 years ago with roughly 40K (6.55-6.8%) in student loan debt, and next month it will be paid off. Average income over those four years was approximately 34K (net). I've also built around 20k in a Roth IRA. I went with a "max IRA/spend the rest on debt" mentality (I had no other tax-advantaged investment options at the time). Hopefully my situations is similar (or at least relate-able) to yours.
I battled the same questions about extra income repeatedly: Should I put it all towards debt? Should I put it all towards investments? Should I have a balanced mix? I ended up with the balanced approach, but one that leaned heavily towards debt repayment. My main focus/tracking points (to keep my sanity) through the entire process were 1) the dollar value of my net worth, 2) savings rate, and 3) cash flow.
Here is why:
1 - By focusing on my net worth, it really didn't matter if I was investing or paying debt; I saw a significant increase in net worth every month. That helped me cope with the "paying debt isn't actually building anything tangible" argument; furthermore, I treated my ROTH as my emergency fund, so that got rid of any need to put money away with a lousy 1% return for an emergency that never came to fruition (I rented and had neither a wife nor kids; what huge financial emergency could I really have anyway? YMMV).
2 - The higher my savings rate was (50%+), the less investment/debt "returns" even mattered, because the time frame was therefore much shorter. There's really no reason nitpicking over a guaranteed 6.8% return vs a potential higher return when you're not concerned with 30+ years of having to make that choice. The differences in returns were negligible over a time frame of just a few short years. Because of my particular numbers, my savings rate made a HUGE difference in my time frame to pay off the debt completely and then put all of that cash flow towards investments, so I focused on my savings rate rather than my return rate. Again, YMMV.
3 - Cash flow was the biggest motivator for me to pay off the debt. I knew that once my loans were gone, that was $450.00/month (~25% of my monthly expenses) in obligations that would go away forever. Knowing that my no-choice-but-to work-to-live slave obligation would be decreased by 25% per month forever was a huge motivating factor to pay off the loans. Keeping that burden for 10 years just to put more money towards investments that may or may not have been more financially advantageous was not in the cards for me.
I was able to pay off my debt quickly, take advantage of 3 years of tax-free growth by maxing out the Roth, completely change careers, and have a little fun as well. At times it felt like I would never get my head above water, but looking back now I wonder why the hell I spent so much time obsessing about it.
Was my plan more psychologically motivated than others? Perhaps. But isn't our "psychology" the number one reason for wanting to FIRE in the first place? We're all trying to get to that point in life where we can enjoy it to the fullest anyway, and to me that seems awfully "psychological" ;) I'd rather enjoy my working years AND my FIRE years, and still make it to the party on time.
As long as you're not spending your discretionary income on cocaine and strippers and your particular "brand of investing vodka" isn't completely ignorant (EX - 100% AA in Pokemon Cards), your advantage in your youth will overshadow any "purely mathematical strategy vs psychological strategy" debate you can come up with.
I hope this helps. Every situation is different, so therefore there is no "correct" answer. However, I feel that my situation my be similar to yours. If not and I've wasted your time, maybe this will help:
https://www.youtube.com/watch?v=c9GU4P-1AWII'll see you at the crossover point; we'll get there sooner than either one of us realizes.