I will go with Raphael on this.
He outclass Robin in Strength,Speed and Durability.
Fighting head on would be in his favour.
Not much in his utility belt that Raphael could not dodge,counter or evade.
The only thing Robin has over him is brains.That would give him a slight edge but not in a random encounter.
I agree on you with the strength speed and durability goes in Raph's favor but as I was thinking on my vote I went for Damian because he has MUCH better equipment. Tasers, knock out gas etc are all stranded gear for a Robin. Skill of the two is split 50 50 where Raph has more in speed and strength but those are none factors if he can't get close enough to use his sai's. Robin can change tactics and take Raph at a distance This would make Raph sloppy as he often gets frustrated when he can't do something, Raph also rely's on brute strength (Big reason why Raph will never be the leader of the TMNT because Leo does use strategy and not just brute strength. This would have been closer if it was Leo and not Raph). This is where Damian's 'brain' comes in to play and uses strategy to win. Raph mainly likes getting in on a fight, Damian can too but thats not his only option so I gave this to Damian solely on these reason's.