On a high level they can be pretty similar. The biggest difference is intent. BitTorrent is about sharing particular files, it's use cases are focused on moving large files across the internet. BitTorrent has an entire algorithm around trading blocks to try and prevent leeching. Supporting the Kademlia (the DHT in BitTorrent) is incentivised by "I want to download things and supporting the DHT is the default on the client and nobody bothers to turn it off", which is amazingly effective.
IPFS is about being a giant shared read-optimized content cache. It is totally ok with leeching because it's goal is to get content to you, not to "share" it. Juan wants to implement "proof of bothering to serve all that stuff" as a cryptocurrency and get rich. The current pinning system for clients of IPFS is a "motivated by it just being the default setting" incentive system for hosting data they want to replace with a cryptocurrency.