Legendary Eirika or Brave Roy?

Better? Probably L!Eirika. But B!Roy is plenty good

You don't really need Eirika if you have a good cavalry sword. She looks strong (I personally haven't leveled her up yet tho), she does have some unique traits and she gives an attack watter blessing, which is also pretty good. If you are satisfied with your cavalry swordsmen, then you don't need to try your luck here imho.

Eirika is powercrept BRoy for galeforce build, so obv she is better.
you can built her based on her brace too as a mixed phase, though she is that durable with 65/59 bulk.