Not to mention that Luthor has years of prep time and explicit weakness exploitation upgrades compared to what IM will have here.
Aaaaaaand also that it only says "prep time" which means Superman is also preping.
Aaaaaaaand Superman has defeated a preped Batman in post crisis.
All that was done in character, not even going all out.
Superman wins both times
Superman has several villains that exploit his weaknesses, and he still beats them.
Even if Tony employs Kryponite or red sun, it's still an uphill battle, but he can do well in the second fight and maybe even win.
First fight though? Not a chance. Superman wins, and the match lasts as long as Clark allows it to.
I don't know about post-reboot Sups, but Pre-Reboot would accidently kill Stark in the first fight.
In the second, well... It depends, I guess, of how Stark use his prep.
If he goes retarded, and just build a better suit, Sups smash his face.
If he uses it to gather informations on the ****er, and build a suit made to beat kryptonians (like, Kryptonite knuckles and Red son energy repulsor and shit), he might wins.
And if he uses it to beg his pal Reed The Man Richards to build him a special ray gun to one-shot him, well, he stomps. Because Reed Richards, man. Reed Richards.