In answer to your original question, it depends. Ultimately, you'd have to calculate how many units/turn you kill on average on each game speed. Since marathon is supposed to have 3x as many turns as standard, as well as policies costing 3x as much, you'd have to kill 3x as many units over the course of the game to gain equal value. Anything more is gain, anything less is loss. This ignores relative value of course. Maybe you can kill more then 3x as many early with raging barbs, which in turn means you'll be further ahead in policies earlier in the timeline.
As for the Aztec sucking, no. Floating gardens are great. Jaguars with a free 2 HP heal per kill are nice, since that carries over. If properly upgraded they turn into barb/unit killing machines that never have to stop and heal. Their UA is nice if you use it. It's one of those UAs that gets increasingly good the harder the difficulty because there's more to kill.
What I do with the Aztecs that generally works like a charm is take Liberty until I get my free Settler and Worker, then take the right side of Honor (the culture duplication from barbs with Aztec/Honor opener is awesome). I will build mostly Jags and try to Rex to 4-5 cities. Barb hunt like crazy and head for Ironworking A.S.A.P. With about 6 Jags, and hopefully some iron to upgrade them, I can usually take out my nearest neighbor (usually a few archers thrown in as well). After that it's smooth sailing. I connect my luxuries, garrison my troops for happiness, build up my cities with floating gardens and play the diplomat with all my resources for a while. I then have the ability to choose whether to go for domination or space race.