I am going to create a new website: Is it better to use the latest major version recently released, or the previous still-supported version? For example, if Drupal 9 were released one month ago, should I use Drupal 9, or Drupal 8, or Drupal 7?
If all the modules you need works on D7, I see no reason to not use it.
That pretty much sums it up. In some more simple projects we are using D7 already. Anything complex, without the budget to invest in helping develop the D7 versions of the modules required, we are still finding ourselves starting in D6.
Worth noting, anything you start in D6, make sure the maintainer intends to create a D7 version and, if not, you have the ability and budget to do so yourself. Sooner or later you'll need to migrate, so select your modules with care.
The major advantages to dev in 7 vs. 6 in my opinion are:
- Drupal 7 has better out-of-the-box accessibility best practices rolled-in (see: http://groups.drupal.org/node/116744)
- When Drupal 8 comes out (whenever that may be), Drupal 6 will no longer be supported. If all the modules and functionality you need are (or soon will be) available in 7, you'll save yourself the hassles of having to upgrade your site if you start with 7.
- Drupal 7 abstracts out the back-end database, so now SQL Server and other back-end databases can be used instead of just mySQL and PostgreSQL. Now that Oracle owns mySQL, that has caused some angst as to what its future will be. Plus if you develop a site for a larger corporation and might need to port it over to "their" backend database, you're theoretically buying some wiggle room with 7.
- It's shiny and new and I'm guessing that most devs will want to develop modules and themes for 7 going forward, so 6 will not get the fancy new add-ons that will be coming out for 7.
- It'll be a faster and easier to get started as you won't have to install a slew of modules just to get the base functionality that 7 already has.
- See #4
If you're uncomfortable with the differences between alpha, beta, dev, RC (and don't know how to use drush) etc then I'd be wary of moving to Drupal 7.
Most modules are still in active development will have multiple releases - the version you launch site with will likely be out of date quickly.
that being said, I LOVE D7 and haven't found many issues with module support :D