I think overall both characters have the same usefulness, but they play quite differently, and that's just thinking of them in general, because as I said the weapons themselves can vary a lot too. Which one I prefer depends on the stage I'm going to play, actually.
Freyja can wield 3 weapons (one of them needs to be a sword) and uses a "Magic System" and a blue bar appears on the right side of the screen. Every non-sword weapon (Bows, Spears, and all the types of Rods and Scepters) need to consume Magic to function, and the amount you need to spend varies a lot and depends on the weapons effects, with the strongest ones requiring A LOT of magic.
In that way, Freyja can be seen as much powerful than Frey, because her weapons can deal extraordinary damage quickly and from a safe distance. But that comes with a pretty big weakness, because the only way to gain back is to attack with a sword or greater sword, and Freyja's swords tend to be quite weaker, she wields greater swords quite slowly which lets her vulnerable, and depending on the stage that it be quite hard to deal with. And the amount of Magic it recovers can be rather low depending on which sword we're talking. And her HP is also lower than Frey's and grows also at half the speed.
So the "3 weapons" sounds like a plus at first, but in truth is more like she NEEDS a sword for the other weapons to function during stages.
So I could say Freyja has a higher potential than Frey but you have to deal with those "downtimes" where you need to be careful to get back to blast things again. A Glass Cannon of sorts that requires some planning.
Frey's weapons are less crazy and more straightforward, mostly for close combat, and he can equip only 2. Frey can swing greater swords faster than Freyja, which is very noticeable. He has also more HP and, if you ever reach the limits on HP, he ends up with double the HP compared to Freyja, so he can be really durable (which is needed if you're at melee range on high difficulties).
The thing with Frey is that, while his weapons do deal less damage overall (Thor Bows are one exception, but good luck with having time to charge the strongest ones lol), he has no limits in how much he can use them, so unlike Freyja he has no downtime or pauses to get Magic back. You can keep attacking the enemy relentlessly, so it sort of balances out the overall damage output between both most of the time.
Also, because Frey's weapons are more direct and simple than some of Freyja's weapons (that sometimes can be unwieldy or hard to use, like the Thunder Spears early on), I think he does a much better job at destroying specific enemy parts too. He has also a much simpler time dealing with large amounts of monsters in close combat with his horizontal swings, because giants tend to surround you. His upwards swings can also send enemies flying, and you can sort of juggle them for a bit too.
There are also little things here and there, like Hammers making him have "super armor" while attacking, so if he gets hit he won't get as much damage nor he will stop attacking.
So Frey to me is more like a "no frills" character with no exceptional features, but also no big weaknesses that you need to play around with. A well rounded character overall, with his potential being tied to how good you're playing instead of the crazy damage you can deal in bursts.
So yeah... To me completely depends on the stage.
