1.
Wowinterface has its own (optional) svn and git repositories as well as the "direct upload" option.
The curse network has it's own svn, git, mercurial repositories as well as a "direct upload" option (on the curseforge side of things only for the last).
The curseforge/wowace distinction is "skin-deep", the back end is common.
Wowi / Curse are completely separate entities though.
As for comparison this is in no way an "objective" comparison or exhaustive enumeration of features so take it as my entirely personal take on things.
Curse: +Localization app, +Author reward program
Wowi: +Much saner design (whatever facilities the site offers, actually work
vs the hit or miss nature of Curse, their track record at breaking search functionality of whatever site they incorporate is impeccable, latest victim being wowpedia), ++Addon user/author community (I realize there's a large cross-section of both but the Wowi administration is just so much better at fostering community spirit and weeding out undesirables the signal to noise ratio of Wowi is vastly superior)
2.
Version control differences are too broad a subject to cover here, in the context of wow addon development most of those differences are inconsequential, main reason to pick one over the other is familiarity pre-dating someone's involvement with addon development or taking the opportunity to familiarize yourself with version control and collaborative development tools in a low responsibility environment.
For the starting author the common-sense approach is "none" (use the direct file upload option).
3.
No (as in "I don't have any favorite tutorials"
)
4.
There is no such central reference point, the FAQ, knowledge base articles, wikis of the different addon hosts might hold some information but it's mostly collective knowledge spread among the brains of different members of the addon author community and site administrators.
5.
Generally speaking setting up an automated or semi-automated process where you can push to both sites from a single point of origin is an exercise for nerds (or geeks, pick whatever term you prefer
)
The easiest way to publish to both sites is to mirror your releases manually.
If you're going to be using the repository system of either site, push your release there, then download the resulting package like any user would do and use the "direct upload" option of the other site.
If you're doing manual uploads to both, just upload the same local file to both.