Same unit size for all bets. Maybe a small spread. Something like 2-5% of your bankroll. It's a grinder's game bud.
The problem with the "confidence of the game" unit sizing is simple: how do you quantify that confidence? If you can't, you're really just making a bet on your confidence. If I'm making one bet, I'd like to operate on as few assumptions as possible -- in the interest of reducing error. However, if you can quantify your confidence fairly accurately, I see no problem in careful unit raises. For instance, you see the Heat line -7, you think they win by 10, calculate the value of those extra three points against the vig and bump your bet some. But never more than 5% of your BR. I can't stress this enough. Variance in all sports is super, super high. Call it what you want to call it (bankroll management, risk of ruin, etc), but they all point to variance: the real killer of 99% of sports bettors.