They have very different uses on a team. You can't really compare them apples-to-apples.
Cinderace thrives on pivot-play. Switch in against a favorable target, probably via Volt Switch or U-Turn. Pummel something for super effective damage. Opponent changes to something favorable against Cinderace. Use U-Turn to pivot Cinderace out. It's inherently high speed also makes it a good revenge killer. This does, however, all but require Cinderace to hold the Heavy Duty Boots, or have a teammate who can reliably remove entry hazards. Cinderace is fairly fragile, and being a Fire type suffers greatly from Stealth Rock specifically, and like most pivot-centric Pokemon from entry hazards generally.
Incineroar has broader coverage and better bulk, allowing it to take more hits and hang in tougher situations than Cinderace. Being a Dark type, most of its STAB options have a lot of interesting utility effects that can be used to manipulate the situation (disable voice moves, ignore stat raises, etc).