[Crazy Jetty]

Mattel seems to be done. So the line is completable. It encompasses DC Superheroes, 20 waves of DC Universe Classics, two years of Club Infinite Earths Subscription figures, two waves of Green Lantern Classics, DC Allstars, Batman Legacy (Two waves), Batman Unlimited, and DC Unlimited. Many of the figures you seem most interested are actually available now. New 52 Batman and Superman, Wonderwoman, Batgirl, Aquaman (All of them new 52)
Second hand prices vary on popularity of the figures.

DC Direct/DC Collectables however has been going since the late ninties (or close to). Sometimes figures come and go and no one ever sees them, then they immidiately start fetching $80 second hand (Stephanie Brown Batgirl is a good example).
The line is huuuuge... and they focus on different artistic styles (One DCD line may focus on a Jim Lee style, another an Ed McGuinness style).
The articulation isn't as bad as everyone likes to whine about. It's usually not as good as DCUC, but they're far from "statues."
I would give a breakdown on prices, but since the Mattel line is at it's end, there's not much point.
DCD/DCC usually is more expensive, since they're sold exclusively through specialty shops, like online toystores and comic shops. So with that line, stores usually hike prices up on popular characters.

It basically depends on where your interests are. But the two lines aren't as incompatable as some may say. Sure, DCD Flash looks rediculous next to DCUC Superman... but if you used a DCD Wonder Woman... the scale would be pretty good. Also they've hit up tons of characters Mattel never did. For instance, many DCD Lanterns look great with DCUC. Brother Warth and Indigo, Star Sapphire Wonderwoman, Sinestro Corps Mongul, Salaak, Orange Lantern Lex Luthor, various DCD Black Lanterns...
Some like Indigo, Warth, Salaak, and the Black Lanterns just don't exist from Mattel. While Wonder Woman, Luthor, and Mongul... their sizes look great in a Mattel display.

So really, it depends on what your personal tastes are, combined with what meets your interests most. DCD will be a much more expennsive line to collect. Mattel though, will likely leave "holes" in your collection.

But ultimately, buy what you like. If you like the looks of a toy, and find it's price reasonable, then chances are you're going to enjoy it.