Even though both are listed as Heavy Duty, they are two totally different types of weapons:

• Versa is a two mode mini gun or rail gun style weapon.
• Hellfire is a close range weapon with a DOT effect.

You should consider your preferred play style; Mid/long range or close quarter if you have to choose between the two. Personally, I would purchase the Versa with credits (since credits are very easy to accumulate) then save up for the hellfire, even though, with that amount of acoins, there are other weapons that do much better.

I have both weapons, neither on Panther. While I think there are better weapons for Panther, I'd tend to agree with above that Versa may be better here. For Hellfire, you need to get closer to the fray and Panther is better off staying out of brawler range. With Versa and your barrier deployed, you can use it as a sniper, and if anyone rushes you, you can change to machine gun and rip them up.

If you can wait for a hellfire crate rush, you can probably get it for fewer a-coins than outright buying it.