For about 300 points you could get 3 War Walkers (~300), 2 Wraithlords (~285), 2.5 Hornets (~310), or 2 Wave Serpents (~320) with 2 Bright Lance.
War Walkers have the most firepower, then Hornets, then Wraithlords and Wave Serpents (which are slightly more expensive than Wraithlords but we'll treat as about the same for this)
Calculated below: The average number of hits from a weapon needed to take down these units entirely (higher is more durable). Note, we're counting single-shot hits here. So a weapon that gets a lot of shots, probably is more effective than these numbers show.
Bolter
3 War Walkers: 3 * 2 * 18 = 108
2.5 Hornets: 3 * 3 * 20 = 180
2 Wraithlords: 3 * 3 * 20 = 180
2 Wave Serps: 3 * 3 * 26 = 234
Heavy Bolter
3 War Walkers: 3 * 3/2 * 18 = 81
2.5 Hornets: 3 * 2 * 20 = 120
2 Wraithlords: 3 * 2 * 20 = 120
2 Wave Serps: 3 * 2 * 26 = 156
Assault Cannon
3 War Walkers: 2 * 3/2 * 18 = 54
2.5 Hornets: 2 * 2 * 20 = 80
2 Wraithlords: 3 * 2 * 20 = 120
2 Wave Serps: 3 * 2 * 26 = 156
Overcharged Plasma Gun
3 War Walkers: 3/2 * 3/2 * 18/2 = 20.25
2.5 Hornets: 3/2 * 6/5 * 20/2 = 18
2 Wraithlords: 2 * 6/5 * 20/2 = 24
2 Wave Serps: 3/2 * 6/5 * 26 = 47
Lascannon
3 War Walkers: 3/2 * 3/2 * 18/3.5 = 11.5
2.5 Hornets: 3/2 * 6/5 * 20/3.5 = 10.3
2 Wraithlords: 3/2 * 6/5 * 20/3.5 = 10.3
2 Wave Serps: 3/2 * 6/5 * 26/2.7 = 17.3
Meltagun (Half Range)
3 War Walkers: 3/2 * 3/2 * 18/4.5 = 9
2.5 Hornets: 3/2 * 1 * 20/4.5 = 6.7
2 Wraithlords: 3/2 * 1 * 20/4.5 = 6.7
2 Wave Serps: 3/2 * 1 * 26/3.5 = 11.1
Note: 2d6, pick the highest, then subtract 1 from the result, minimum 1, has an average of 3.5
So, what does this tell us?
Well, we see that against small arms, the good armor saves and extra wounds of Hornets, Wraithlords, and Wave Serpents are good, taking sometimes 2x the amount of punishment. Against specialized anti-infantry weapons, everyone does worse, and against the Assault Cannon, the War Walkers in particular seem underwhelming. Against specialized anti-tank weapons, the War Walkers are actually slightly (against plasguns, lascannons, etc) or significantly (against Meltaguns) more durable than Hornets or Wraithlords, due to the Power Field. However, the clear winner for durability in all scenarios are the Wave Serpents, which run a bunch of wounds, have the Serpent Shield, and have a solid base armor save.
So, War Walkers: surprisingly durable, but also surprisingly vulnerable to anti-infantry guns. Wave Serpents: highly durable, but less firepower than the other options.
In terms of "platforms for Bright Lances" it seems like Hornets could be a good choice. Compared to War Walkers, they have somewhat less firepower, are somewhat better against anything that's not dedicated anti-tank, and somewhat worse against anything that IS dedicated anti-tank. They pack more of a punch than Wraithlords or Wave Serpents do for the same price. More mobile than War Walkers which is the killer here.
War Walkers seem like the winner for raw damage output of course, and hold up surprisingly well to Lascannons, Plasmaguns, and Meltaguns compared to Hornets and Wraithlords, point for point. That vulnerability to small arms fire might be a problem though, and they can't move and shoot using CTM like Hornets can.
Wave Serpents and Wraithlords both have less damage output, but bring other things to the table. Wave Serpents have the lowest damage output (since they cost 5% more than the War Walkers with only 4 Bright Lances instead of 6), but they are, well, Wave Serpents. You're paying for that huge durability and the transport stuff. They're not really fire support-only tanks. Worth it if you need to transport things, in my view. Wraithlords can't transport anything, but have extra guns for up-close combat and are good in assault. Excellent in the right army. Always on the move, in all likelihood, so even more problems with that damage output.
I think all four of these platforms have their place. War walkers with the most damage output, Hornets with the maneuverability, and Wave Serpents and Wraithlords with somewhat less output per points, (and the Wraithlords more durable vs small arms, less via antitank) but bringing other things to the table.
Also, it should be noted that Wraithlords aren't taken in squads so your opponent may assign too many lascannons to it and waste fire.