Shield tanks have a lower maximum EHP than armor tanks. Shield tanking allows you to fit more damage mods to your tank while increasing its EHP. This allows you to build 'Sniper Tanks' - tanks with very high Alpha damage that can break other vehicles very quickly. However, they are softer than armor tanks, and will be brought down by AV weapons faster.

Armor tanks have a much higher EHP, and benefit from repairing modules that restore armor much faster than their shield variants. These tanks are very durable, but they are slow, and easy pickings for a shield tank fitting. Hope this helps,

PS

Guys, you have to look at the damage types vs. the inherant resistances of the tanks defense. in new eden we deal in 4 damage types:

EM (Electromagnetic)
Thermal
Kinetic
Explosive

Shields have increased resistance to kinetic and explosive damage while armor has increased resistances to EM and Thermal. Now take a look at the current ingame weapons and their ascociated damage types based upon the stated tech they use.

Laser Rifles - EM and Therm (yes, people use them to great effect vs shield tanks for this reason)
Forgegun - Kin and Therm (based on it being a railgun type weapon thus placing it in the hybrid class)
Blaster turret - Kin and Therm (also placed in the hybrid weapon class)
Railgun - Kin and Therm (hybrid still xD)
Swarmlauncher / missile turrets - Kin and Exp damage
AV Grenades - Kin and Exp
Flux Grenades - EM (possibly Them?)

that being said, look into the damage type your using vs what your trying to combat. in all reality, armor vs shield doesnt really mean anything as a well thought out AV assault using the proper damage types and tactics will take out anything. yes, armor tanks are much tankier, but their also much slower, while shield tanks cant absorb as much punishment, but have a bit more agility to compensate allowing them to "dance". also, shields regen, armor doesnt. its these balancing factors that make no one tank type superior to another, it just depends on the skill of the tanker vs the skill of the infantry hes facing.

another thing on weapon types vs other weapon types, if we look at the way new eden weapon systems work, all weapon groups have a longrange and short range variations.

long / short
Rail /blaster
beam laser / pulse laser (laser rifle / scrambler pistol)
artillery / autocannon

long range weapon types generally have high alpha damage, but poor DPS, where short range weapon systems require you to get closer, but have much better DPS in general (balanced by their short range nature) so basically:

armor tanks more hits, shield is more agile and has natural regen
rails beat blaster at range / blaster beats rail up close
missiles are just funky like that

EDIT: im sure my grammar and spelling is just plain absurd, please be nice :)

now that i think back, your right in that missiles are "most likely" only one damage type. thank you for pointing that out :)

i would assume that the missiles fire kinetic damage due to their prevailance on caldari tanks and caldari styled launchers, and mechanics wise, it would make more sense if they were kinetic, that way, their still less effective vs shields, but not completely useless like they would be if they were explosive

EDIT: on the forgegun note, the description states that its based on rail tech, so id be REALLY surprised if it was doing EM damage... especially since the only weapons that do EM/KIN dam are projectiles.... at least as far as EVE is concerned. i would imagine that they would not mix up their formula for damage types now. hence my assumption that forgeguns are Therm/Kin

2nd EDIT: Hybrids have always been a nice middleground weapon, being relatively effective vs armor AND shield, but not proficient vs either one or the other, hence their KIN/Therm dam

Missiles do explosive damage so yes they are more effective against armor. But it has just to many drawbacks like slower rate of fire then railguns, less accuracy, very slow flight time and with that easy to evade (well at least the large missiles). Personally when you really insist on using a shield tank with a large blaster turret then put 2 cyclic small missile launchers on it aswell. Some extra firepower against armor tanks is allways usefull.