Hard to say because I've never spent much time playing the 18's.
What i've found is, I think alot of the 'bad' players in the 27's are the ones who were unable to be winning players in the 9plyr. sng's & prob. figure they've got a better shot in the 27's. Not sure this is the reason but do know there are a bunch of bad players in them 'but' at the same time there's a handful of regulars, some of whom are profitting very nicely in them (some of them will also be playing the 18's.. others mix in the 45's). I've seen quite a few micro 27sng players with very high ROI%, 3x higher than a good STT-SNG player (seen players with avg. buyin $5 who have over $15k profit!!).
On Stars you'll see a bunch of them who are the regs. in the 27's (reg. speed) & same with on Fulltilt.
If you're good at making adjustments when tables get short & get used to them, 27's can be quite profitable to play in. Bottom line though (imo) you're gonna have to be good either way, no matter what you're playing if you want to be one of the ones who are doing well in them.