Regarding how LaTeXing's closed/open-source status figures into the comparison: In late 2017, LaTeXing was made free (as in beer), with a promise that it will soon be open source:
[LINK REMOVED AS OLD SITE IS ABANDONED / HIJACKED] The code is open source on GitHub and should be available via package.io (see below). For old documentation see wayback https://web.archive.org/web/20180329210203/http://docs.latexing.com:80/
(In late Sep 2017, this page says: "In the next couple of weeks the source code will become available and the community can continue to improve the package together.")
The Package Control web site lets you track the popularity of various Sublime packages. The following search URL will show you the total downloads for both LaTeXTools and LaTeXing (clicking on the name of each package will show the download history):
https://packagecontrol.io/search/latex
Right now, LaTeXTools has nearly 3x the total downloads of LaTeXing, and LaTeXTools has been about twice as popular in recent months. Perhaps the planned move of LaTeXing to open-source is motivated by this.
The posts here from 2014, and the few blog entries I've been able to find comparing these packages, note that LaTeXTools lacked command completion and project management. However, the current LaTeXTools documentation indicates both capabilities have been added.
So far I have done only a simple comparison, with a single .tex file (i.e., not a project), on macOS Sierra. Both packages have a "Check system" command, and all necessary tools were located by both packages. LaTeXing had no trouble building the file and automatically launching the Skim PDF previewer. But I could not do an inverse search from the PDF back to the .tex source. I spent some time looking online for a fix, but didn't find anything helpful. LaTeXing has a GitHub page, just hosting an issue tracker, but this issue wasn't addressed there; in fact, there is very little developer activity there. In contrast, LaTeXTools worked fine out-of-the-box, with the source/PDF link working in both directions. Also, I'm finding the LaTeXTools documentation to be better, and there is much more activity on GitHub. For now, I'm sticking with LaTeXTools.
