They seem to have the same philosophy and to be pretty much the same thing.
In an old post it is claimed that TinyCore is based on Slitaz, but TC's main developer rejected this claim. However, he admitted that TC was (partially) inspired by Slitaz.
Now, the question is: after all the years, how have the two distros differentiated themselves?
Why would one pick one distro rather than the other? (apart from TC's more recent kernel, which is just one difference)