Are there any differences between the two for using with PHP?
Which is better (lighter, snappier)?
Can I install Eclipse plugins in Aptana?
How good is Aptana beta 3 and does it have jQuery, HTML5 and CSS3 support?
Does Aptana have code folding for PHP (braces closing)?
Not that I'm any expert :), but I have used Aptana and Eclipse for years now, including my current setup which entails a stand-alone Aptana install for Ruby, Python, PHP and HTML 5-y stuff as well as an Eclipse install with the Aptana plugin for Java/ColdFusion work. I wanted to respond with my thoughts on your questions. I hope they are helpful!
If you care to, check out Titanium Studio. In January, Appcelerator purchased Aptana. This is the 'new' Aptana Studio and contains all the same features and functionality of Studio 3 but also adds in Appcelerator's Titanium Developer mobile SDKs and features. It's pretty cool!
I hope this helps you out a bit!
Best, Craig