AP: Both scales to the same feat. However, Chopper has better showings for his strength. With Heavy Point he blocked Dosun's hammer swing, who could take a casual kick from Sanji. Chopper hurted his hand a bit but because he chosen a bad Point to counter a hammer, according to him. While Usopp one-shotted Daruma after getting enough preparation. Chopper can also increase his advantage with stronger Points, since Heavy Point is his standard human form.
Horn Point, Kung Fu Point and Monster Point are all upgrades to his regular AP.
Chopper takes the AP game.
Durability: Chopper also gets this. Even if they are comparable, Chopper has his amps available. Guard Point would counter a good amount of Usopp's gadgets and his Dura scales to Big Mom, so Usopp needs to use something like the Banana Shuriken or explosives to prevent Chopper's fur from absorbing the blunt force of the projectiles. And I think I remember Guard Point bouncing away from an explosion in Drum, but I might be wrong.
Range: Usopp hands down. No need to explain this.
Stamina: While both are pretty tough, Usopp is a monster in taking damage. Chopper would need to kill him in order to truly put him down if Usopp is determined enough in winning the fight. The sheer tenacity of the long-nose is something Chopper can't truly imitate.
Mindset: Both are cowardly and tend to panic quite easily, but Usopp is better at getting himself together and turning the tides of battle. He's also more tactical and once he calms down he gains more through sheer intelligence. And Chopper would fall for any bluff Usopp makes as seen in the manga quite consistently.
Chopper's no idiot however. He has shown tactic skills and he doesn't play around in a fight, since he knows how dangerous it could be if he loses his powers mid-fight. Added to the fact that he can come up with a plan if needed like in the Mirror World.
Abilities: Chopper has the skill in CQC. Usopp can't feasibly win if Chopper manages to get close enough to make him lose the range advantage. However, Usopp's versatility is by far superior to Chopper's. He has a lot of different gadgets to aid him in the fight.
Haki is not a factor. Usopp has only awakened it during a crisis and hasn't shown any proficiency with it outside of that unique instance, so he's not getting any advantage from it. If Usopp loses track of Chopper during the battle, the latter might be able to approach without being noticed. And Chopper has his nose to locate Usopp, unless the latter counters this like he did against Luffy with rotten eggs or similars.
How do I think this'd go?
They start at Usopp's range so he gets a big advantage at the beginning of the fight. Usopp is prone to hide and snipe, but he can't hide himself from Chopper's nose. He'd need to land a Egg Star in Chopper to mess with his nose. Any other option is useless since Chopper will locate him no matter where he hides.
Central Park is the battlefield here, meaning Usopp can and will use the trees and rocks to hide and snipe Chopper. The reindeer knows Usopp and will not let his guard down to give him any advantage. He will know where he's hidden and will go after him the moment the fight starts. He'll be cautios enough to watch out for any traps and tricks Usopp could pull off.
Usopp's best option here is to null Chopper's nose with Egg Star. Chopper would dodge, parry or block most attacks considering he's physically superior in every aspect (I forgot to talk about speed but it's an easy win for Chopper too). If Usopp lands it, he'll gain a massive advantage and will be able to set up traps for Chopper.
If Chopper gets close he'll pummel Usopp with ease, but if the latter sneaks a convincent lie (and with Chopper anything is convincent) to divert Chopper's attention he'll get the advantage to run away or land the Egg Star.
I firmly believe Monster Point is enough to put Usopp down, but it's a coin toss. Chopper would need to be sure about using it when Usopp couldn't run away. Otherwise, he might lose the form and will get incapacitated, losing the fight.
This comes down to who can gain the advantage first. Chopper is superior in every physical aspect, but Usopp is smarter, more resourceful and tougher.
Both know each other's skills to some extent. Usopp's full arsenal isn't known by Chopper, but he knows that in order to win he can't fall for a lie or let Usopp land something that'd null his biggest advantage. He saw the fight against Luffy. He knows how dangerous Usopp can be if given the chance.
I'm voting Chopper High-Diff. He's simply superior in stats and could anticipate Usopp's moves. Once he gets close enough he'll not leave him run away to gain the range because that'd be a terrible situation.