Finn vs Abel

I have actually just recently started using the 5* Abel I got as pity breaker not long ago and have been positively surprised with his performance.

Anyways that is not what you are asking for but Abel has the benefit (or bane) of having variable IVs and higher availability for merging which matters on the Arena.

As units themselves, Finn has 1 point higher in Spd, Atk and HP. He also has 7 points moved from Res to Def, compared to Abel. Overall, due to brave users being player phase, Finn is better.
However, he can only reach +1 right now, whereas Abel can reach +10. Abel can also have IV's, which provides yet another advantage over Finn.
If you just want a normal unit for Arena Assault, use Finn. If you think you may end up merging, or have a +Atk or +Spd Abel, use him.