So I've been on a Star Wars kick this past month ever since Rogue One came out; been rereading the Timothy Zahn trilogy for the first time in, like, 25 years, listening to the soundtracks in my car, replaying the Jedi Knight series, will probably replay the KOTOR's next, and I've been thinking about giving The Clone Wars a shot but need a little convincing because hey, the prequels are garbage. I do watch Rebels and enjoy it well enough, and its OT setting is what made me jump on board in the first place, but from everything I've heard, TCW is better, despite the PT setting. Are we talking DS9 vs VOY better here?
I wouldn't compare either to DS9 or VOY as it'd be somewhat of a false analogy. These aren't two shows worked on by separate crews, it's pretty much the same crew on Rebels as previously worked on TCW.
The main difference is that TCW is an anthology show, whereas Rebels focuses on a regular cast. That changed as the show progressed and around season 2 & 3 became more centred of the core cast and moved away from stand alone episodes towards 2-4 part arcs.
There's some fairly major stuff in here that informs some of the larger elements of the lore. The Mortis episodes and the Yoda/Dagobah stories in particular really get into the crazier cosmic mystery elements of the force.
As is typical of a lot of good shows, the first season a a bit rough going, Both in terms of storytelling/characterisation and on the technical side. It's actually quite jarring to look at the quality of the earlier shows compared to what they were able to start pulling off from season 3 onward. It's almost like a whole different show.
But if you still need convincing and aren't too worried about spoilers (though if you're up to date on 'Rebels' there's not much, if anything here to spoil) then I'd just suggest you have a look at
this. It's a pretty good overview and gives a sense of the breadth of the show.
-Is it worth watching the 2D Clone Wars series first?
Worth? Certainly. First? Not necessarily.
Though the 3D series did inherit some design and voice actors from the 2D show, they're not technically in continuity with one another. Honestly though, there's very little that directly contradicts and they don't cover much of the same ground.
-What's the best way to go about watching it? Just take the plunge and buy the complete series set, or are there any (non-torrent) online alternatives?
I think it can be legally streamed on a few different services, which may be the best and cheapest way to go.
The advantage of streaming with an anthology show like this is that you can pick particular episodes and arcs and (at least at first) skip the poorer quality ones.