INV Pros:
+ Has very high resistance against the two most common forms of damage: smashing and lethal.
+ Has a "God Mode" in Unstoppable which caps resistance to all types of damage except Psi for 3 minutes.
+ Self-Heal/HP Boost in Dull Pain.
+ Taunt Aura in Invincibility.
+ Can get decent defence when herding (though it's less likely for a scrapper to do this than a tanker).
+ Has repel protection in Unyielding.
INV Cons:
- No resistance to fear, -recharge, slow, confuse or endurance drains.
- Other less common types of damage can cause major problems for INV when unstoppable isn't available.
- Very, very weak against Psionic damage.
- No endurance buffs, can be fairly heavy on endurance when running all the toggles at once.
SR Pros:
+ Defense based, so you get hit less often by debuffs and mezzes.
+ Strong against nearly all damage including Psionic since the defense is positional - melee/ranged/AoE.
+ Gets Slow/-recharge protection and a Self-recharge boost in Quickness, which can free up extra slots.
+ God Mode in "Elude", same idea as Unstoppable except it's defense based.
+ Click-based mez protection, so it won't detoggle if you get mezzed.
+ Tends to be able to dodge sapper's endurance drain beams.
+ Scaling resistance buffs in the passive powers that will increase your resistance as your health drops.
SR Cons:
- No endurance buffs, can be fairly endurance-heavy running all the toggles at once.
- When you get hit, you get hit HARD. Autohit damage, Toxic damage and high-tohit buffs will hurt you badly.
- You start panicing every time you see a DE quartz eminator or someone suggests doing a nemesis mission.
Both benefit from taking the medicine pool power "Aid Self", I'd say SR practically requires it.
INV benefits a lot from taking "Tough", to combine it with its already high S/L resistance.
Thank you ever so much!
Maelwys covers all the points as usual 
thePhilosopher Martial Arts/Regen/Fire Scrapper
theRegulator Empathy/Energy/Soul Defender
Total Inertia Ice Blast/Kinetics/Psy Corruptor
Total Inferno Ice Blast/Thermal/Leviathan Corruptor
My basic idea is a samurai. I'm thinking either Super Reflexes or Invulnerability. I simply refuse to take regeneration becouse it does not fit my concept.
So, my question is. What are the pro's/con's of the scrapper shields?