Well, since this is open again, I guess I'll give my vote.
Rex has better gunslinging abilities. That much we already agreed on. However, thanks to the mind-reading additions, Roland effectively has precognition due to being able to see Rex's next move before the latter even knows he's moving. He'll know where the guy is going to shoot before it happens and will be able to react accordingly. Roland's instinctive reaction means there isn't a whole lot else that's going to surprise him, either.
There's a good chance Roland (and maybe Rex too, depending on what his tactics are like) will try and seek cover if neither of them is blown to hell within the first two seconds of the fight. If that happens, Roland has the advantage. He can easily locate Rex using his enhanced senses and his tracking/trailcraft skills before the latter has a chance to see him first, and he'll know if Rex is observing him at any point thanks to his extrasensory abilities that allow him to tell when he's being watched or followed. Roland also has him beat in actual battle strategy; The man was already outsmarting John Farson's men by the time he was in his late teens and was leading troops in battle before he had even made it to thirty, and since then he's been travelling Mid-World and killing people, beasts and robots for over three hundred years. I should really rewrite his intelligence section at some point, because what's currently there really doesn't do him justice.
Actually, another thing that's never really occurred to me before now is that Central Park has a fuckton of trees in it, along with some isolated statues, at least one boathouse, numerous restroom areas, an entire ******* museum, and plenty of other things to hide behind. You'd have to start the match with them both standing in Sheep Meadow in order for them to have absolutely no chance of getting to cover before the opponent snipes them. (SBA says that won't happen, by the way. Sheep Meadow doesn't span several hundred meters, which is how far apart SBA puts them in this fight.)
But, anyway, I think Roland wins in the end. They're both capable of one-shotting with their weapons, but Roland's mind-reading abilities give him a leg up over Rex's trickshots. (Would he even be capable of ricocheting bullets off trees without them getting embedded in the trunk somewhere?) If the fight lasts long enough for cat-and-mouse tactics to come into play (which it should), Roland's got Rex beat thanks to his tracking skills, superhuman senses and far superior battle experience, on top of his other advantages he has right out of the gate.
