Leo or Eldigan?

With the way the game currently works, ranged units are completely useless. Eldigan is one of the best units in the game right now, but if you only have a few orbs I would recommend saving them for a new banner. Since Eldigan is only 1/6 focus heroes.
The prince banner is not really worth pulling from at all.

Ranged units being completely useless!? Tell that to all the Lindes and Takumis out there. Seriously, that statement is wayyyy out of line. There's a reason why people consider Takumi and Linde as top tier heroes. They hit hard and from range, meaning they can possibly take 0 damage during combat against a non-ranged unit.

Aside from that, I suggest you go for Eldigan. Leo is not a viable mage in the current meta. Leo's base SPD is at 22, which means almost virtually everyone can double you and his ATK is at a base stat of 43 which is not enough to give him a OHKO potential at all. His weapon can be used to kite people but that rarely helps you out even if the situation presents itself. If you want a red mage, Sanaki is a better pick. She can OHKO Hector for Pete's sake! She's also from the Siblings Bond banner so it's a win-win if you get her instead of Eldigan anyway.

Contrary to what yobnomekop saids, Eldigan is not "one of the best units" in the game right now. While his base stat is high (182), his kit is mediocre. The only real reason to get him would be if you want to achieve a higher arena score, but aside from that, he's not necessarily that game changing compared to those at the top. His passive "lunge" is a suicide mission when used. Often times, he will be far apart from his team, leaving him vulnerable to the opponent's team. Although debatable, I suggest you take that passive off during your arena run, as it will only limit his survivability. Fury on the other hand, is actually a good passive, as it gives you a 12 stat boost in exchange for 6 damage per turn. Because you gain +3 DEF & RES, it's more like you're taking -3 damage per turn, which is not that bad. However, this can work against him as losing -6 HP after EVERY combat (and yes, I do mean after each encounter he has with others) will increase his likelihood of being KO'd as the game goes on. Regardless of how much damage he takes, he is guranteed to lose 6 HP, and so even if if the opponent supposely does 0 damage to you after to two turns, you still lose 12 HP. Fortunately, you cannot kill yourself with Fury so you can still live with 1 HP.

Take a look at his stats. (All neutral)
45 HP/ 51 ATK/ 27 SPD/ 37 DEF/ 22 RES

Because he has such a high base stat total of 182, his stats are pretty good, but they way they are distributed show some clear weaknesses. His SPD is on the low end, so it's likely he will be doubled more than half the time by other units. His base ATK of 51 is solid, but it's not sufficient enough to be considered a OHKO. It would usually require either two independent turns or some assistance from teammates to secure a KO on an enemy. His base DEF of 37 is extremely high, almost like a tank, and so you can expect him to take multiple hits from physical foes without too much worry. Most physical units deal around 45-50 ATK so even if he does get doubled, he will survive. His high base HP is 45, which is also a plus too. Though, his base RES of 22 is fairly low, making him susceptible to any magical damage, and with his low speed, it's likely he won't be able to survive after combat. So overall, he's a DEF tank with high HP and moderately high damage but suffers from low SPD and RES.

His special takes 4 turns to charge (because of his weapon), making it fairly inconsistent in the arena. While not impossible, I wouldn't rely on it too much to help pull you the win. Even if it does trigger, it does not deal a massive amount of damage for a 4 turn move, but it can definitely help when it is activated.

Honestly, if you're looking for a good red unit, the princess banner is the way to go. They have both Eirika and Lucina as focus heroes and they are both a solid pick for any team composition. I'm sure you have already heard about how good those two units are so I won't go on any further. If you still insist on choosing between Leo and Eldigan, I strongly suggest Siblings Bond since it also has a chance of giving you a better red mage (Sanaki), just in case you don't get Eldigan from your pulls. That's all I got to say.