Hi James,

Quite a difficult question to answer in my opinion. Your point about the functional documentation doesn't matter to much, there isn't any good documentation anyways. Both the old and new API are still available so that is nothing to worry about. The bigger thing to worry about is the bugs and especially the differences between the 'basic' Odoo V9 and the enterprise. I think your first thing to do should be to check out if everything you need is in the default Odoo V9 or if you will need the enterprise version. If you'd need the enterprise version you would have to pay an extra 20€/month/user extra, which might be to much for you.

From a developer perspective the major changes are that @api.one is no longer supported and neither is the xpath for strings. Migrating code from V8 to V9 might give you some troubles / extra work along the line too. If you can deal with a few bugs and that not everything is 100% I'd go with version 9. Why? The bugs can be patched or your Odoo base can be upgraded from Github. So you'll get those bugs out without too much problems. Imagine migrating data from V8 to V9 and all your customizations though.. You will have to both have quite some knowledge in database migrations and you'll have to migrate the code which will result in changes too. If you let Odoo migrate you'll have to pay for it or have the enterprise version.

Conclusion: V9 will probably cost you the least in the long run but you might experience some bugs and problems in the beginning. First check if what you need is still in Odoo 9 (and if you want the enterprise version)

Yenthe