It doesnt really matter. You SHOULD/MUST/WILL transition to 6* champs in a couple of months. Like 3-4 months max. All 5* are trash. Like 4* and 3*. Dont think too much. You can close your eyes and blindly put 5* ag on anyone and it still shouldnt make a differance in 3-4 months. P.s give AG to anyone who you NEED to progress in story mode. Thats your only goal. STORY MODE!
5* champs can still be plenty useful if you don’t have them in 6*. Pretty much all story and EQ content can be done by a well played top tier 5*, although obviously it is more challenging than with a 6* R3/R4.Specific to the OP’s request:My personal POV is that Scorpion is the more broadly useful champ, although AM has lots of helpful utility also. AM requires more skill to fully utilize. I also think he benefits much more from his sig ability than Scorpion does, despite my liking Scorpion more overall. So, if I just had one AG to use and I would use it on Absorbing Man.