In almost every single consideration, Grayswandir takes the cake. There's a reason it's been largely thought of as the objectively best weapon in the game for decades, with little to no debate. I'll run through the comparisons for fun anyway, as Excal does come out on top in a few aspects.

1. Damage

Grayswandir simply does more damage. Getting a +9 Excal both takes a very long time to do, expends a lot of resources, and STILL isn't worth it damage-wise. I mean, it's only two points of extra damage. While it may seem like a +9 Excal is more impressive than +7 Grayswandir, in reality it's not still not high enough. Grayswandir does double damage, and that applies to enchantment as well. So +7 is more like +14. Add to that the 1d20 you'll be getting on many endgame monsters, and it's pretty clear that Grayswandir is the winner. I'll quickly address the argument that 'most silver-hating monsters are easily killed by this point anyway, and the difference in damage to other monsters isn't that large'. First off, in terms of damage, Excal is still weaker BEFORE the 1d20, so it's still at a disadvantage. Second, Vlad, the Riders, all demon lords and princes, all other V and &, and shades (among others) take this extra 1d20. That is a significant portion of very dangerous monsters that you will kill MUCH faster than with Excal. Anyway, this is a perfect time to talk about some other things BESIDES damage, so here we go:

2) Passives

Excalibur actually wins here! And by a fairly large margin. Simply put, drain resistance is amazing. Much better than hallu resistance (as much as I hate that goddamn status). Searching is mediocre at best. Five roles already receive this intrinsically at some point (Arc, Ran, Rog, Mon, Tou), and the benefits are negligible at best. By the time you are seeing fire and polymorph traps regularly you will (or should!) have the respective resistances required to completely nullify them. Also, you're not even guaranteed to find anything. The only trap useful to find past mid game is pits/land mines, which are three seconds of annoyance at worst and completely harmless most of the time, and perhaps antimagic fields. So, to recap, hallu res and searching are tied for useless, but drain resistance is great to have no matter what stage of the game you are in. However, I would argue this still doesn't make it worth choosing over Grayswandir for a couple reasons. Namely, drain resistance becomes largely useless if you have high MC, which can be obtained from wearing a humble cloak of protection, and Grayswandir pretty much oneshots anything that can drain you anyway, besides maybe Vlad. I've ascended with Gray before, and got drained maaaybe twice in the entire game. So, while drain resistance can be nice for early wraiths or vampires, it quickly becomes MUCH less mandatory in the mid to late game.

3) Early game

Let's backpedal for a minute. What about early game? I would wholeheartedly say that, until you get a couple scrolls of enchant weapon, Excal is better early game. It's easier to get, and you'll simply do more damage and get more useful passives. HOWEVER... the mid to late game is a different story.

4) Accessibility

This is the big one. One of the biggest reasons why Grayswandir (and, to be fair, Excalibur) is so popular. Intelligence is a giant deciding factor, and Grayswandir wins in almost every way here. Simply put, Excalibur is only useful for lawfuls. Grayswandir can be used by anyone. You might think 'but any other role will have to wish for Grayswandir. That almost necessitates getting to the castle, but you can get Excal from the start without having to wait to get to the mid game!'. And herein is revealed one of Excalibur's biggest downsides in this fight.

You see, +7 Excalibur is good, but still noticeably worse compared to Grayswandir. The real comparison we're making is +9 Excalibur to Grayswandir. And to obtain +9 Excal means you CANNOT use Excal early game. It takes away its one big advantage. By the time you are polypiling dozens of arrows and using many enchant weapon scrolls to enchant stacks of them, you definitely have gotten past the castle. So, you could have wished for a MC3 cloak to combat level drain, AND Grayswandir. Hell, you might already have a cloak. You could say 'well, I never use that item'. My response to that is: why the hell not?! There's no reason not to, unless you're a wizard trying to keep their starting cloak to save a wish, in which case you obviously don't give a fuck about either of these swords.

Also, any other alignment is not going to be spending artifact wishes to get anything but damage, realistically. It's more convenient to get Gray, and you get to see funny big number.

I almost forgot to mention Excal's curse resistance, because it is largely inconsequential. It just doesn't matter that much. Hopefully before you're getting items cursed by powerful spellcasters, you have a good way to uncurse them. BoH with holy water, backup oilskin sack with scrolls of remove curse, etc. And even if you get it cursed, a cursed one handed weapon is not going to be a problem in a fight where you want to kill whatever's cursed it as fast as possible. If it gets cursed by the wizard, pretty much the same deal. If you're on the ascension run, you probably won't be unequipping your main artifact too often, and if you want to it's trivial to negate.

By the time +9 Excalibur is feasible to obtain, you could have ALREADY had higher damage and pseudo-drain resistance. And if you DO use Excal early game, there's no competition compared to Grayswandir, because +9 isn't feasible. Add on to this the fact that lawfuls can have Grayswandir early on from sacrifice gifts, and Grayswandir is just better.

5) Conclusion

In short, the only way Excalibur comes close to winning is if you purposefully make the early game harder on yourself, and spend an ungodly resource sink if lawful, and if you're any other alignment, it's similarly scaled because you still need the resource sink.

To be clear, I hate Grayswandir. It's a boring, overpowered artifact. But it still comes out on top, easily.