Quick answer: Lightning Cheaper, quicker, smaller target, more DPS/DPM with the main cannon(s). The Vanguard is actually really only worth it with 2/2.
^^this Lightning is very mobile, Vanguard - when you going against few tanks, then you need gunner, and probably another Vanguard to cover each other
Lightning, AP, 2x zoom with Rival 3, but you cant take on a tank alone. If you are in a group, indirectly at the front with several other tanks, thats where you will gain a benefit with the Vanguard with its better armor and damage output over shorter time and just can bring a Halberd to get a "random gunner" to assist you, which is better than not having a gunner. Enforcer sucks if the random can aim. Instantgib infantry vs "always" seeing them run away etc. If you really want to go solo in a Vanguard you can bring the canister gun, but then you can just as well get a Lightning instead and just aim at inf with the AP. 2x zoom matters allot in this regards. NOTE: That I am talking about them fully upgraded. Overall, the cheapest cost vs gain is the Lightning. If you have other characters as well I really recommend DBC buying the Lightning AP AND its 2x zoom. Saves you the hassle to unlock on all characters.
It depends really, If you are playing more mobile, then you'll want the Lightnings superior speed (forward and backward) and acceleration. If you are likely to want to shoot infantry, then you'll want to take the faster firing Lightning. Lightning also has a smaller profile, so it is easier to find a "hull down" position for the Lightning. Lightning is 100 nanites cheaper. However Vanguard is better fighting vehicles, with its main gun having higher alpha and velocity, and more EHP (and the shield). Harassers are a major problem for the Lightning, less so for a Vanguard, especially if there is backup nearby. Vanguard also has a secondary weapon, giving you more options (Walker to deter ESFs, Canister to deter infantry) with seat swapping.
depends on your playstyle: if you are an aggressive/offensive player then Lightning is better since it's faster, is more agile, has faster reloading main cannon, has more damage output per second and has a lower profile to make you less probable to be hit. generally, these traits are most effective if you can make use of them by being quick and "dodging" income fire. also, higher firerate means less punishing for missing shots which is very important in a 1on1 standoff. if you are a defensive/support player then Vanguard is better since it's better armored, has the invincibility shield and has more ways to fend off an attack by seat-switch (say between AP cannon and Halberd) as that makes you immobile. you are clearly better off with it if you rather like to keep your distance to enemies and "snipe" them with the main cannon instead of rushing and overwhelming them. dont get me wrong, you can still use a Vanguard aggressively, but it's theoretically less effective due to the Vanguard's slower speed, acceleration, agility and firerate.
