MCH question

[Jeykama]

So, should I get the chest or the gun?

Depends.
Is this a static or pug situation? In a pug I might be more inclined to buy the coat since the coffer is FFA.
You get a guaranteed weapon someone can use every week in the form of the coffer - do you get free lot on those (especially for an alt job)?
If the brd/mch coat drops, would you get free lot on that?

Personally, I never got the coat to drop but it's easy enough to get a coffer in my static now that all our mains are kitted, so I used my books on the coat.