I've upgraded 7 TWiki installations to Foswiki.
As of now (November 2009) it looks as if about 200 bugs are present in TWiki
which have been fixed in Foswiki. Not surprising as Foswiki now has about 15
active developers -- to 1.5 for TWiki after the great corporate lockout.
So the featureset is currently with Foswiki -- and the future is clearly with them.
Besides, the switchover is no more difficult than doing a version-to-version
upgrade (they are the same codebase). And the new Foswiki way of doing future
upgrades is a snap. Unpack a special tarball and you're done. After previous
painful TWiki upgrades, it's a snap (they copied this technique, by the way,
recently-- but who knows how long they'll be around to keep it up -- or if
they'll ever find the manpower or community to make the backlog of bugs go down).
Big companies are switching or have switched. Yahoo, which has the world's largest TWiki installation, has a hiring opportunity open for a Foswiki developer to spearhead their migration and continued customization. Why Foswiki? Because that's where the features and bugfixes are coming from -- it's the only active community driving innovation.