Compare full price, discount and used game costs.
Games are entertainment purchases, so cost per hour of play is the comparable measure — a long game at full price can be better value than a short one on sale. Working out the sale price, the used price and the hours you realistically expect turns three options into one number each, and the cheapest route is whichever gives the lowest cost per hour rather than the lowest sticker price.
Cost per hour of play
Sale price = full price x (1 - discount/100); cost per hour = price / expected play hours
Comparing against other entertainment helps: a cinema ticket is a few units per hour. Games routinely come in far below that, which is why long titles represent strong value.
Compare the two cost-per-hour figures here. Digital sales often undercut used physical copies, though used copies can be resold afterwards.
Community completion-time data for the specific title is far more reliable than your own optimism, especially for games you may abandon early.