It's hard to say which will have better legs. Man of Steel and especially Batman v Superman had terrible legs, but Suicide Squad had good legs and Wonder Woman had phenomenal. DCEU films are all over the place in terms of word of mouth. And since Suicide Squad is (IMO) the worst DCEU movie, yet had the 2nd best legs, I don't think it's a foregone conclusion that quality will be a strong indicator of legs.
On the MCU side of things, several recent Marvel movies had weaker legs than anticipated. Spider-Man is a notable exception, opening lower than most people thought but then having good holds through the summer. Thor in particular had decent legs for the first movie, about as good as Doctor Strange and Winter Soldier. The Dark World though ranks among the worst, better than only IM3, Civil War, and Ultron. The legs of MCU films seem to revolve around whether or not the movies meet audience expectations. You see IM3 and Ultron having some of the worst legs and you can probably imagine why - both movies pulled the rug out from under audiences regarding villains and were considered by most to be a big step down in quality (although those who don't like IM3 are categorically wrong, I have proof). Civil War was overly long and far less focused on "war" than people thought it would be. It turned out to be mostly a straightforward Captain America sequel with about a half hour of Avenger vs Avenger action. And it's no wonder Thor 2 had poor legs. Movie so forgettable that people forgot to tell their friends and coworkers to check it out.
Reaction/hype for Thor seems higher than Justice League at this point, but it's also closer to release and the overwhelmingly positive reviews have already hit. I haven't been watching closely, but reaction to Justice League trailers seems good, not great. Guess we'll know more about that when we get the inevitably legendary review thread.
Tl;dr - I dunno lol ¯\_(ツ)_/¯