Hi Jon, I've also been considering a new editor. Personally I use Eclipse with PDT - but this can use a bit of resources. There are lots of other options:
Komodo http://www.activestate.com/komodo-edit
PHPStorm http://www.jetbrains.com/phpstorm/
Sublime text 2 http://www.sublimetext.com/2
Coda 2 http://panic.com/coda/
Vim even... http://www.littlehart.net/atthekeyboard/2012/05/17/how-the-testing-sausage-gets-made/

In my case, there are some core features that I can't really do without so I basically wrote those features down and when I try out other editors I try and get those features up and running.

Sublime text 2 here love it, and textastic on iPad

I can also vouch for Netbeans. Works better on a beefier computer, and you'd need to do some heavy customising to colourise the Silver stripe template file sy tax correctly, but code completion is fab and it also handles version control nicely.

When I worked at SilverStripe I was introduced to TextMate (macromates.com)

It's a Mac application, so if you're on PC, bugger.

It's roughly $30 NZD to buy, well worth the investment if you're serious about productive development. Key features I enjoy include:

- SilverStripe Bundle, custom SilverStripe functionality built in to TextMate, enabling faster SS Specific development, Templating and Debugging.
- Go to file (Your entire project is indexed, unlike others I've used in the past)
- Xcode tools

Outlined on their website:

Ability to Search and Replace in a Project
Auto-Indent for Common Actions Like Pasting Text
Auto-Pairing of Brackets and Other Characters
Clipboard History
Column Selections and Column Typing
Completion of Words from Current Document
CSS-like Selectors to Pinpoint the Scope of Actions and Settings
Declarative Language Grammars for Graceful Mixing and Hacking
Dynamic Outline for Working With Multiple Files
Expand Trigger Words to Code Blocks With Tab-able Placeholders
File Tabs when Working With Projects
Foldable Code Blocks
Function Pop-up for Quick Overview and Navigation
Plug-able Through Your Favorite Scripting Language
Recordable Macros With No Programming Required
Regular Expression Search and Replace (grep)
Run Shell Commands from Within a Document
Support for Darcs, Perforce, SVK, and Subversion
Support for More Than 50 Languages
Switch Between Files in Projects With a Minimum of Key Strokes
Themable Syntax Highlight Colors
Visual Bookmarks to Jump Between Places in a File
Works As External Editor for (s)ftp Programs
Works Together With Xcode and Can Build Xcode Projects

I can knock out complete SS websites in under 4 hours now, including design time (low budget, but fully fledged sites nonetheless). This wouldn't be possible without TextMate ;)