Gotta agree on their games today and keeping talia, but it's absolutely valid for people to think hamling has more upside than talia. Hamling is a much better athlete- his combination of closing speed and leaping ability would almost rival wood (and he actually bears a lot of similarities to wood). Generally, his intercept marking is very good, as is his spoiling. When hamling gets the ball, he is far better at setting up attacking plays than talia.
Talia is stronger (useful, but not so important when our defence is mostly based on interception), has better endurance and is better at providing an outlet option. Over time, hamling could improve his ability to provide an outlet option a lot (easton wood didn't get much of the ball early in his career either), and his disposal and speed could make him a far better attacking weapon than talia. I believe hamling has a much higher ceiling (generally what upside means) than talia, because talia will never have the same speed, leap, reach or disposal that hamling has, whereas hamling can potentially develop more in most of his weak areas.
Being a DFA is pretty irrelevant- geelong should never have delisted him.
It's a bit of a moot point anyway, because both can definitely play together. If anything, their strengths complement each other, as talia can engage an opponent's body while hamling floats across the front to mark. Neither should be traded, but it's certainly valid to think Hamling might be better long term.