Pay off the high-interest credit card debt first. The math is pretty straightforward here - if you're paying like 18-25% interest on that $15k, you'd need to find investments that consistently beat that rate just to break even, and most people aren't timing the market well enough to make that work. You're basically guaranteeing yourself a "return" by eliminating that debt, whereas the stock market has no guarantees.
Your friends who say you're missing out on market gains aren't wrong that timing matters, but they're underestimating how much that credit card interest is working against you. Every month you carry that balance, you're throwing money away on interest that you could redirect toward actual wealth building later. Plus, debt has this psychological weight that makes everything else feel harder. Once that's gone, you'll actually feel motivated to invest instead of just thinking about it.
After you knock out the debt, then you can be aggressive with a brokerage account. You'll have better cash flow, less stress, and a clearer picture of what you can actually afford to invest monthly. A few months of focused payoff now puts you in a way better position than spreading your money thin between both.