Legitimately a very tough one, I have both right now.
Dunkley - Higher ceiling, he's a couple of injuries from 120 capability and probably should push 110 regardless just from being fit again. Bevo factor and Treloar factor are both negatives and durability has been shaky. Dunkley full time forward would be a mid 90s type, so there is downside.
Ridley - I think can push 110 this year but I'd be surprised if he went much beyond that and he doesn't have the same ceiling. Intercept, kick-in and primary rebounder combination is fantasy gold and a consistent role, Sicily, Ryan and Ridley really the only 3 that do it. I think Heppell is more of a retainer in the area than Saad who tended to gung ho run into and out of and into trouble before kicking it long, Heppell is going to look sideways first and second to the more penetrating players, Ridley is the top of the list of those. Kind of similar to how Simpson operated alongside Docherty at his peak. Be surprised if Ridley is below 95 barring injury.
Perhaps the biggest factor between these two though for me is the rookie they're going to be pushing off the field and out of the side. The back rookies look putrid right now, I'd say there are at least 5 forward rookies I'd want on the field or in my team before getting to Highmore as my top defensive rookie. I'd say that Henry and Jones, near the bottom of the barrel, are equivalent to Kosi who is the clear 2nd best back rookie right now.
That rookie factor gives Ridley the edge to me. If both score 110, which is what I'm hoping for having picked them, then Scott for example is another 10 points on Highmore for mine.
Pure upside play though Dunkley has more hurt factor to not have, he could easily be the top forward by 10 points a game this year, Ridley I think is going to be one of about 4 guys in that 105-110 range in defense.
I'm not sure I've helped you at all here