Between the two, I like pod gun more, but they're both good options. I enjoy playing both. Wait for other opinions, but I would suggest:

1. If you want to keep the disc launcher on guardian, then viper and pod guns on aegis are both fine. Aegis can stay out in the open longer which is useful when playing viper, so for the versatility in your armoury I'd suggest viper 8.

2. Otherwise you can consider pod gun on guardian and DL on aegis (which will help aegis with one its achievements as well.. the guided weapons one.. if you care about that).

They're both for credits, IIRC, so really it's a question of what to get first. Eventually you should consider getting both, and then ditch the DLs unless they're working well for you.

I would personally go for pods BC vipers have too much exposure and your in enemy line of sight which equals get murdered by tetra rifles that you can't even see.

 Sure, there's the shield for aegis but that means that you're stuck waiting for 10s for your shield to cool down before being able to fire again. 

Pods however are curving and can hit ppl around corners and behing walls if used well. Plus splash DMG and you could put it on teammates so if they die enemies also die.

 Cool down for both pods and vipers are close enough but pods are faster. 

There are very few mechs that I know of that work well with vipers. And most are legendary. Pod guns should work well with almost any mech.