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Good question, Bertram. I’d (almost, see note below) always recommend using the latest version of the library. The addition of .on() is only one change of many since 1.4, there have been an absolute ton of additions and improvements over the last few releases.
Besides, once the libraries are minified and gzip’d the file size difference will only be (by my best estimate) about 6 or 7 kb.
The only time I’d be wary of going with the latest version is when updating the version of jQuery used in an existing project. Though updating is almost always painless, the chance of something breaking in your existing code when moving to a new version of jQuery is always present. When I’m using the Google Libraries API to load jQuery (which is almost every project) I will always specify the exact point version I’m loading rather than relying on it to just feed me the latest version. When a new version of the library gets rolled out I’ll switch to the updated version in a dev environment and make sure nothing’s broken before updating on live.