Paul,
The 1.8 was the only one avaliable when I built mine. Actually I started out with a ST3000. Huge mistake, lots of deadstick landings. Out came the ST and in went the Moki. If the 2.1 is just a bored out 1.8 I'd go for that. As far as ZDZ I've never had any experence with gassers. I just prefer the simplicity of glow and the compactness.
Jeremy