SPY vs low cost index funds

Hello! I have a quick question about index funds. I was told by an investor at one point to throw half of my investments towards SPY and half towards QQQ. Since then, I have maxed out my Roth IRA on half SPY/QQQ and I have made over 20% return this year. My question is whether I should invest more in a low cost index fund since I will be able to buy more shares per investment? Is it worth it to have more shares of a low cost IF or an IF that has a higher annual return? After maxing out the Roth account, I contribute close to 3K/mo into a brokerage account that is also going towards those 2 ETF’s plus a couple of play investments. Just wondering if I should continue doing what I’m doing or if I’m setting myself up for failure. Thank you!

When it comes to these types of index funds, they all generally perform the same:

By “low cost” I assume you mean a lower share price… buying an index fund with a lower share price won’t make a difference. They will all grow by a similar percentage so what’s more important is the dollar amount.

Hope that helps!