JSON polyfill: JSON 2 or JSON 3?

I was wondering the same thing. JSON3 says:

Changes from JSON 2

JSON 3...

• Correctly serializes primitive wrapper objects (Issue #28).
• Throws a TypeError when serializing cyclic structures (JSON 2 recurses until the call stack overflows).
• Utilizes feature tests to detect broken or incomplete native JSON implementations (JSON 2 only checks for the presence of the native functions). The tests are only executed once at runtime, so there is no additional performance cost when parsing or serializing values.

In contrast to JSON 2, JSON 3 does not...

• Add toJSON() methods to the Boolean, Number, and String prototypes. These are not part of any standard, and are made redundant by the design of the stringify() implementation.
• Add toJSON() or toISOString() methods to Date.prototype. See the note about date serialization below.

bestiejs.github.com/json3/

HTML5 Please says:

JSON 3 is a polyfill compatible with nearly all JavaScript platforms. It is a drop-in replacement for JSON 2 that uses feature tests to detect broken and incomplete native JSON implementations, supports asynchronous module loaders and has-aware optimizers, and is built to specification standards.

JSON 2 is an older, alternative polyfill by Douglas Crockford.

HTML5 Please has updated their JSON page to include JSON 3 as the main polyfill for use, with JSON 2 taking a backseat as the older replacement.

Use json2.js if you want to improve the performance of your page in IE7 (and possibly 8). I was happily using json3 for months and then I saw this question and did some research. All of the jsperf's performed 1-2x faster with json2.js in my IE7 VM.

One example:

http://jsperf.com/json-vs-jquery-json/2