I've been giving this some thought since the disappointing showing both had at SDCC.
Both companies have their weaknesses and strengths. Bad sculpts, paint jobs, breakage, scale trouble and the price range. Not all of these problems happen too often outside the price problem for DCC. Even so I have to say DCC is looking the worse of the two. They finally worked on their breakage problems but had a smaller scale for DC Icons. They were improving the line and offering more selection. Then they cancelled most of their line and started a new one in a bigger scale. With the same old character choices and it still has a steeper price range.
Mattel seems to have a good character selection in terms of doing waves with related characters (ex. the bat family) mixed in with tv shows and characters from other DC franchises. They do obscure ones like the Reaper and new characters like Batwing/Luke Fox. But instead of doing separate lines for the movie stuff it's taking up space and I can't say I was pleased with the wave of the same old characters we got coming next year. I was hoping they'd finish off their bat family lines with Red Hood, Spoiler, Cassandra Cain, Tim Drake and even a new Nightwing. They have a Bizarro but it's basically a Superman variant with a new head and accessories.
I'm not thrilled with either company but like I said, at the moment DCC is the worst one. They keep cancelling things, have quality control issues, the same repeated character selections and are priced too high. By now we should have a number of characters completed for different franchises within the DCU. Yet we're getting another out of scale reboot. I'll give Mattel more points for doing more obscure characters but what about the main comicbook series?
Justice League: Both companies do characters from the comic series all the time. Mattel does tend to spread them out more but their also doing an entire collection for the movie.
Titans/Teen Titans: Mattel did a number of them in their old line though some need better updates. The same could be said for DCC when they were DC Direct, which was a while ago. Neither have really done anything for the Rebirth designs except for doing Damian figures and an upcoming Nightwing.
Green Lanterns: Hal and John are in DC Icons and Hal will be in Essentials for DCC. Mattel is doing Jessica and Simon. Kyle and Guy need some love as older figures will cost you more.
Aquaman: He has figures coming out for both lines, Mera in in the Mattel movie wave but none of the other characters have upcoming figures. Dolphin was just shown in Aquaman so it's doubtful she will appear anytime soon or Aquagirl.
Superman: Yup on both accounts for the man of steel. Jon has one coming out in the Super Sons pack. Lois has no current figure, nor does Superwoman/Lana, Steel or the new Chinese Superman. While Bizarro is coming from Mattel it's not the RHATO version which is extremely disappointing.
Wonder Woman: Yes for the lead for both lines, comic and movie versions. Donna hasn't had an update in awhile and it's still iffy if Cassie will have one released. Artemis was last released as a variant for Wonder Woman by Mattel years ago but a RHATO figure would be welcomed. Mattel was planning a WW exclusive line that hasn't been mentioned since it was announced that would have included Dr. Psycho.
Flash: Barry has upcoming figures, neither Wally or Bart has any figures planned. Which is weird since DC made a big deal about red headed Wally's return.
Batman: This is the best covered but there are still characters missing and scales are off. Mattel has done the most with covering all the current DET team except Stephanie, Cass and Tim. Jason badly needs a proper comic based figure as the one DCC wasn't that great.