Okay, so, I had a thought. Primaris and Death Guard have been the poster boys for this edition (yes, I'm aware, Undivided has the spotlight right now) and I decided I'd compare their basic heavy infantry Troops options.
First off, I decided I'd go over their basic toughness (this stems from a discussion I had with That Guy at my FLGS over whether Plague Marines were roughly equivalent to Intercessors). Let's use the basic Space Marine boltgun.
Let's assume a Tactical Marine fires into each from out of Rapid Fire range (one shot). He makes his Hit roll and we come to the Wound roll. Now, Plague Marines already have the advantage because of their higher Toughness. The Tactical Marine has a 50% chance of making his Wound roll against the Intercessor, but only has a 33% chance to wound a Plague Marine.
Even if the Tactical Marine makes his wound roll, each unit has a good save, of 3+, which is a 33% chance of failure. The Plague Marine has a Feel No Pain (Disgustingly Resilient) of 5+, which gives an extra 66% chance of failure. However, the Intercessor has an extra wound, so it's fine if he takes a hit.
Let's go over their weapon options next. Intercessors are laughable as far as their weapon options go. They have three options total, and the whole unit has to take it (no piecemeal). The default is the Bolt Rifle, which has a respectable -1 AP, strength 4 and Rapid Fire 1. Basically a better version of the classic Boltgun; it has 30" base range (15" for double shots) and looks great in conversions. Then you have the Auto and Stalker variants, each of which shifts the stats of the Bolt Rifle in one direction or another. The Stalker gives an extra point of AP and gives extra range, but it's only Heavy 1; great for sniping and ObSec, but awful for getting in close. As far as I know, Stalker Bolt Rifles are also free points-wise right now. On the other end of the spectrum is the Auto Bolt Rifle, which has no AP and shorter range but has Assault 2, and is also the only one which costs anything.
On the same note, their melee weapons are somewhat lacking, being that only the Sergeant gets anything better than basic attacks. The Sergeant can take a Power Sword, though, so it kind of makes up for it. (Unless you're a Blood Angels player. If you're a Blood Angels player, you're stuck with a chainsword instead for some reason. Don't ask me why.)
Plague Marines, on the other hand, have decent weapon options. Their basic boltgun is lacking (basic boltguns are awful, after all) but they get options galore. The Champion can take either bolt or plasma pistols or take a plasma gun instead of his boltgun (always take the plasma gun, always). In addition, they can have two of their number replace their weapons with either Assault or Heavy versions of the Death Guard-style flamer, a blight launcher, a melta or a plasma gun. All in all pretty good at range.
In melee, they're even better. In a squad of five, you can have all of them take different melee weapons. The Champion can take a power fist and a plaguesword, which gives you options as to how to deal with your enemies. In addition, two can take bubotic axes or maces of contagion, which are special Death Guard melee weapons, and another two can take Flails of Corruption or Great Plague Cleavers (always take the Flail). The Flail has overflow wounds, plus D3 melee attacks instead of base, which makes it a powerhouse in melee. However, that's balanced against the low Attacks characteristic of Plague Marines (a hilariously low 1, 2 for the Champion), making them hard-hitting melee attackers if you build them right.
All in all, I'd put the Plague Marines above their Primaris counterparts for weapons options, and roughly equal for toughness. That extra wound goes a long way, usually.
Note we're using the Death Guard version of the Plague Marines for this, not the Undivided version. I feel that the Devout Marines (Khorne Berzerkers, Plague Marines, Rubrics, Noise Marines) need to be taken as a Troops option to be really worth it; there's better Elites options out there by far. Intercessors are a Troops option for everyone that can take them, on the other hand, making them more ideal.
In conclusion, the current edition's box infantry is actually balanced toward Chaos, but not by much; weapon options go a long way, but Intercessors are Troops regardless of who takes them. They're about as tough as each other, all in all, and I'd recommend either for your Troops slots in each respective army. I will say that weight of fire is more effective against Plague Marines just because only one shot needs to get through to drop them, regardless of context.