Between Titan M1 and Mammoth M1, which I have both, which should I invest in for later?

I have only M1 Mammoth, M1 Firebird, and M1 Shaft. I don't know what other things to try and invest in. Should I invest in Twins, Vulcan, or a different other gun like Isida? Should I get Viking or Dictator, Hornet or Wasp?I am really confused, since I know once I get my M1s, I probably won't be going back and upgrading non-main weapons until later. I am fairly tight on crystals.

What about an interview with a really good gold box hunter? Sure there are articles, but it would be cool to hear about it from one who actually goes around gold box hunting a lot.

Great on soooo many levels :D