So, the fight almost certainly would start as a result of Edward looking for Shirou as Shirou would be a prime target for holding a potential philosophers stone. Edward is also certainly the one in charge of the fight with the ability to use alchemy to control the battlefield. Shirou can fight at much longer distances and can use enhanced jumping to create that distance but Edward has a dozen ways to provide defense against Shirou's projectile swords and block vision with dust clouds or create traps. The infighting portion of this is where things get rough. Ed could certainly rush down Shirou but between reinforced organs and a blindingly fast reaction speed most attacks are likely to miss or fail to hit a killing blow, and Shirou's reinforced weaponry is likely on par with Ed's carbon powered northern mail if not stronger. Who wins is almost a toss up with me favoring Ed with two major factors being the deciding points for me.
Does Alchemy work in a Reality Marble?
A reality marble is honestly to specific and rare a phenomena and each is totally unique in its conception and creation so you can't really give a general answer. The question is really "Does Alchemy work in Unlimited Blade Works." Since as I covered above a reality marble is just replacing reality around you it would matter how close to the real world your internal world is, the biggest concern being that there would be no connection to the energy powering alchemy below the earth, followed by concerns of rebound from not correctly identifying what the surrounding materials would actually be. It would be 100% certain he could use it inside of Rider's marble since Iskandar is literally recreating a place from the real world. It should also work in UBW. Now this isn't because I think I think that UBW is close enough to the real world, but because it is literally the ultimate forge and a place of creation. If alchemy couldn't find something to work on there then I have no idea where it would ever find it.
The Result
Aaand none of that matters. Provided Shirou lives long enough to pull Unlimited Blade Works out the fight is over. His already fast attacks only grow faster from there as he no longer has to actually project the present swords and no amount of walls or defenses will overcome the sheer weight of fire Shirou has once the reality marble is active.
Also of major concern is Shirou's ability to understand the working of Ed's automail arm and disassemble it through firepower, something he wouldn't even need UBW for. A real concern as it is the most consistent way to remove Ed from a fight in the series and one that Shirou is easily capable of.
I do think that out of ten fights Ed would probably win three. He is perfectly capable of ending Shirou with a good shot, but the circumstances for that are tough to pull together.
- He needs to hit him with something totally unpredictable, hard to do since in the UBW route Shirou repeatedly blocks surprise attacks from Medusa and also dodges her attacks in the Fate route as well.
- It would need to hit him hard enough to break Rho Aias and... well I'll be blunt there isn't a single punch thrown in FMA that hits hard enough to knock even a single pedal off short of perhaps a god tier Father without being weakened by the reverse circle or Von Hoenhiem if he were to throw some really strong Philosophers Stone backed attack out.
- It would need to do is be an instant kill shot past the steel like organ defenses. Because Shirou tends to remain conscious through a lot of trauma and if he lives long enough to retaliate then Edward is done for.
Finally in fighting style Edward is something of a trickster fighter, he often works on analyzing an opponents abilities and thinking his way around strong foes and retaliating to attacks rather then going for aggressive rush down when in a fight without prep time. That means more often then not Unlimited Blade Works is going to be summoned or Shirou traces and destroys the automail and that is the game. And of note is that all of this breakdown is
before you take into account things like genuine magical artifacts and the like the Shirou can pull out.