I'm about to start a Google map based project and am wondering if the release version of GMaps v3 has most of the features that are available in v2, or if it would be best to stick with v2 for now. Is there anywhere that has a list of features in v2 that are not built into v3 yet - and features of v3 that are not in v2.
Personally, I'd avoid using v3 for live sites until it comes out of Google Labs unless you gain a significant advantage from the v3 features.
APIs in Code Labs may change at any time as they evolve. They are also not committed to the 3 year deprecation policy, and Google's obligations with regards to testing and bug fixing are more lightweight.
That's not just theoretical. The change of the get_ and set_ syntax only ran in parallel for a few weeks. Because v2 is a mature product, syntax changes that break large numbers of existing pages are avoided. If you do go with v3 you have to remain attentive to the possibility of future page-breaking syntax changes.
V3 releases end up having to be pulled back significantly more often than v2, that could be partly due to lightweight testing, but also because there's no equivalent of a v2.x release. v3 code goes directly from "Google internal testing" to "live on all v3 sites" in one step.
I'm using v3 for hobby, I've made a small project which allows you to add markers, measure distances, add polys, save maps and realod... I've found that v3 is faster than v2 with many markers (less than 100, without any clustering), and I've found simpler to add markers and customizing the icons. But, often, the guys in google changes the rules... so if you're starting a real job, I suggest to use v2, or you will have to always check if everything works.