There are a bunch of differences but the important ones (imho) are:
Dat doesn't use immutable addressing (addresses stay the same when content changes) while IPFS does.
Dat at the lowest layers is stream-oriented, allowing stream-oriented services and applications that are near-real-time. IPFS is static blob/object oriented.
IPFS has a better developed "discovery" network at present (if you use Dat today you are typically in your own island whereas with IPFS you're part of "the" IPFS network). This is being worked on however.
Dat doesn't use immutable addressing (addresses stay the same when content changes) while IPFS does.
Dat at the lowest layers is stream-oriented, allowing stream-oriented services and applications that are near-real-time. IPFS is static blob/object oriented.
IPFS has a better developed "discovery" network at present (if you use Dat today you are typically in your own island whereas with IPFS you're part of "the" IPFS network). This is being worked on however.