JLA:
Superman
Batman
Wonder Woman
Martian Manhunter
Flash
Atom
Black Canary
GL- Kyle
Red Tornado
Captain Atom
Green Arrow
Firestorm
Plastic Man
Vixen
JSA:
Alan Scott
Captain Marvel
Black Adam
Flash-Jay
Dr. Fate- Hector
Hawkman
Hawkgirl
Jakeem Thunder
Mr. Teriffic
Power Girl
Sandman
Stargirl
Atom Smasher
Dr. Mid-night
Hourman
Wildcat
vs
Avengers:
Captain America
Thor
Hulk-Classic
Vision
Iron Man
Wonder Man
Hercules
Ares
Sentry
Ms. Marvel
Moondragon
Hank Pym
Namor
Dr. Strange
Hmmm. I think the DC teams have too many street level guys that could be one shotted by the Marvel teams. Marvel looks like their team is overall more durable.
I think the JLA/JSA win a lot based simply on teamwork. The Defender's heavy hitters are infamous for not working together, while the JLA/JSA's can work together like a well-oiled machine.
I think they have a few more heavy hitters in there too.
The Flashes would be very hard to counter. But I will say Marvel's team has some sick higher tiers. Two of their sleepers I think would be Dr. Strange and Ghost Rider btw.
I don't think we know how powerful he really is. Dr. Fate would be a good counter or maybe the Thunderbolt who can do pretty much anything. Though, Hector will be have his hands full with Strange.
I don't think so, the other team's supernaturals/mages will serve as counters, and each side has some that are very good at anti-mage work.
People like Thor, Wonder Woman, and Captain Marvel are physical bricks and can be fought by physical bricks, but they all have a ton of magic resistance and are going to stop spellcasters from dominating.