How do you think about fees when comparing index funds with similar holdings?

Two funds can track nearly the same index and still end up with very different results over decades. The gap often comes down to the expense ratio and any trading costs the fund passes along, which can look tiny in a single year but compound quietly over time. I've noticed many beginners focus on the headline return and skim past the cost line. Looking at fees over a five or ten year window, rather than a single quarter, tends to make the difference easier to see. It also helps to check whether a fund's tracking has been close to its index, since a fund that lags its benchmark by more than its fee is worth a second look. Does anyone here compare funds mainly on cost, or do other factors like tracking accuracy, fund size or tax treatment weigh more heavily in your own process?