If you love the free Cave Story and are in a position to give something back, getting CS+ is probably an easy way to show your support~
If you've never played CS before though, then CS+ might be a better choice due to having more features for the original game. Not to mention the soundtrack and art differences...
Not everyone jumps on the retro look right away, so the new art and music might help convince one to play~
Then there's the whole Steam community thing... If you have a bunch of friends you could all play CS+ and compete for achievements/scores and chat about it...
But, if you're just buying the game for Wind Fortress... I wouldn't consider it a selling point after playing through it. Don't get me wrong, it was fun... but it's just an extra challenge.
I'm more interested in all the free mods the community has developed... [available for the free PC version]