If I was arming Plauge Marines, I'd only ever take meltaguns. Meltaguns are cheaper than plasma and I find them more versatile. Melta can instant kill T4 characters (like most marine heroes and wulfen) and are fantastic against both vehicles and high toughness creatures in general. Also, going with instant death, single wound models with T4 or less, will be denied Feel no pain as well.
Plus, you can charge after shooting melta, which only creates more options.
I find that the ability to quickly take out vehicles (drop pods, knights and the like) is crucial for victory, and the meltagun helps with that more.
Never discount the need to charge to take out the last few models in a unit, or to contest/control and objective. I don't want to have to be stuck not charging, because I spent the points on two plasma guns.
The two shots on plasma is nice and all, but I'd rather have AP1, and armour bane.
For champ equipment, he's striking last anyway. If not giving him a power fist, he should at least have a melta bomb and maybe a combi-melta too.
Plasma pistols are too expensive for what they are, the same cost as a plasma gun, but not as good. I'd rather have a combi-melta for 5pts cheaper (which can then also let you get a melta bomb too).