AMFPHP all the way. Finished a project recently using PureMVC and AMF
http://www.growersaunaturale.com/
Although Zend is officially supported by Adobe, I seem to recall AMFphp was written by the same developer ??and has some speed -performance advantages as well
As for debugging remoting projects, can't recommend the java based tool, Service Capture enough
http://www.kevinlangdon.com/serviceCapture/
AMFPHP is cool, but there are a few caveats everyone should now.
1. Yes Wade Arnold manages both AMFPHP and ZendAMF. However, AMFPHP has been in beta for over 2 years, and since Wade took over more than a year ago there have been 0 updates, while ZendAMF has received several.
2. Yes ZendAMF is supported by Adobe, which is probably why it is getting updates while AMFPHP is not, ZendAMF is backed by Zend, it won't be going to the wayside.
3. Debugging can now happen with my browser at
http://www.zamfbrowser.org. While things like Service Capture and Charles are nice, those debuggers require you write service implementation code, and they do not generate AS3 code for you.
4. Upgrade to PHP 5.3 and see what happens to AMFPHP. Unless you are either a.) willing to maintain AMFPHP beyond PHP 5.2 or b.) willing to not update your server from PHP 5.2, you are stuck with the current server setup and will not be able to upgrade unless you accept one of the two conditions.
So although AMFPHP might seem like the more attractive and comfortable option, ZendAMF does have its advantages. Personally, I'd rather go with the solution that is actually getting updates and that is officially backed by both Zend and Adobe. This is why I thought it was absolutely crucial that it have a Service browser, so I made one.
In the end, its always the developer's choice what they go for, I'm just lettin' ya know why I chose the way I did.