Basic difference is A.geroldi end as up as blue adults as a sling they have a blue carapace and legs but a red striped abdomen characteristic on the slings on many avic species.
A.versicolor start out as slings that are completely electric blue and and end up as red fuzzy adults with a blue carapace.
I can't speak about A.geroldi care but versi slings are a little tricky they require high humidity with high ventilation. Also avic slings in general tend to be sensitive to enviromental conditions. But once past a certain size they become hardier.
If this is your first avic sling, I would go with the A.versicolor because they are less expensive, so if it dies while you are working out the kinks on avic husbandry you haven't lost much.
It is possible to get both, lol, but I really only wanted these Ts
female A. avic -Chula
female B.boehmei -Blaze
B. smithi -Monarch
female B. smithi -?
~~~~~~~~~and two I got as freebies (slings) B. albop, and G. aureos
I love my geroldi's. I can't wait for the adult colors to come in. I bought 4 slings last year and hope my luck holds out to get at least one male/female to try to breed. It's nearly impossible to find adults for sale so you kinda have to get slings and raise them up. Last time I saw an adult female for sale it was around $350. Slings go for about $35-50 depending on size.