The core CFML tags and functions are well supported by all 3, with mostly the bleeding edge features that separate them along with the support options if you’re concerned with needing expert help if you find a bug or quirk that becomes a showstopper.
The other consideration is the platform you have to host the CFML engine. Adobe ColdFusion is well supported by hosting companies if you’re not hosting it yourself, but the others have limited options at the moment.
UPDATE: I just read that OpenBD can run on the Google Apps Engine! Live demo! Look at the appserver value. (It’s not available on there yet… work in progress…)
Most of my projects are self-hosted so I don’t care about the platform, I just need stability, easy-to-get support (free or paid-for), and a promising development roadmap.
I’m not quite finished… each CFML engine have their own unique features. Take Railo with its CFVIDEO tag for example, for someone that could be a deciding factor. Creating YouTube sites could be as easy as pie. Open BlueDragon works natively with Amazon’s SimpleDB and can pull & push files to/from Amazon S3. ColdFusion 8 has built-in MS Exchange support (which is cool) and AJAX stuff which I don’t really care for. jQuery is the way to go. :-) But ColdFusion 9 is scheduled to be released by the end of the year and will arguably leapfrog the other two with a ton of new features.
I haven’t been too happy with the support from Adobe when I discover bugs and their 2-year product cycle means some issues don’t get addressed for a long time, although their intermediate patches are warmly welcomed.
A price to pay…
Then there’s the pricing issue. Sure, £6000/$7500 isn’t a massive price for big companies wanting to run with CF Enterprise, but small companies or personally financed start-ups can’t chuck that sort of money into application software when it costs the same amount to purchase new server hardware and host it for 4 years. Even on the enterprise level if you need to expand a cluster and whack in an extra web server it’s £1000/$1500 for the hardware and then £6000/$7500 on top. A large organisation would still ask why it’s costing that much to pop in an extra server.
The current economic climate has put pressure on many businesses to cancel or scale back on projects and the awful £/$ exchange rate has made CF a lot more expensive in the UK. So maybe now is the time to look closely the open source CFML engines.