I like upgrading whatever team support I've got first. Depending on the crew my bro and I run with, ammo comes second. If someone commits to ammo, then I leave it alone. I generally run a riot shield and a sentry or vulture. Those are the last things I upgrade, if at all.
We had a pretty good lobby tonight. We hit every challenge. I'm not sure if the other sniper (for that challenge when it came up) did this, but what I have been doing lately is to do Team Support then Weapon Specialist or Tank (we've been doing pretty good running this one ... I'll explain it in a minute) then my pistol. I do my pistol only enough to allow for two primaries - for the strict purpose of being able to take that second primary for whatever challenge comes up.
With just me and my bro playing, I've been running Tank a lot. The ARX seems like a crap gun, but the ROF and fast reload compensate for lack of WS. The ARX especially helps if you can find Rapid Fire, Grip, and a Muzzle Break. We usually do. The think with Tank is that, once fully upgraded, you've got 200% health. It also come in handy if you do, perchance, run out of ammo. One of us usually runs Armor and the other Feral Instincts. We'll have those fully upgraded right away - before we even touch ammo.
Activating traps, dropping support and/or ammo (if you're the one responsible for it), and playing a gun game ... we run max points a lot. A heck of a lot - even when we've got weaker players in the lobby and are throwing them $$ left and right.
You know what gets me? These guys that won't listen when it comes to the Venom X and traps. Everyone makes a mad dash for the Sticky Flares and Pipe Bombs. Man, those are nothing but eye candy. I mean, they can help. But, for example, the other night we were getting two Rhinos hypno'd ... per Hypno trap. My brow figured he was activating new rhinos when he threw his trap down, but I don't think that's the case. I think you only get two hypno-creptids at a time. So his two hypno-rhinos were cancelling mine. Regardless, they were a HUGE help.
But, yeah, if you equip your Venom X guns first, then create a trap for each player, there's still enough left over for at least one person to make flares and a pipe bomb. But everyone has to be on the same page.