I think Shields can benefit from the damage mitigation that lies in Knockout Blow especially from lvl 30 onwards. How much damage you achieve actually does depend on how you play the sets. In all round terms I would fully expect Fire to trump SS in damage even in terms of doing a set amount to an endurance bar but you're the builder of the character.
He will honor his words; he will definitely carry out his actions. What he promises he will fulfill. He does not care about his bodily self, putting his life and death aside to come forward for another's troubled besiegement. He does not boast about his ability, or shamelessly extol his own virtues. - Sima Qian.
Footstomp, best power ever 
Combined with that +dmg power and rage footstomp does really nice damage, added that most SS attacks have knockdown abilities.
The downside on rage, the crash gives a -def portion (afaik), and as shields rely on defence that might be very tricky. Luckely footstomp still does knockdown
Now, its just waiting for handclap to be as the mastermind pet, a knockdown instead of a knockback.
Damage wise i think they got par, FM has some lovely single target attacks, only with rage i think SS can ditch out more.
50)Sinergy X/(50)Mika.
(50)MaceX/(50)Encore
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Got a Shield/SS tank myself and can only say: Love it. Much more fun than my old fire/fire tank. 
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I have a 50 Shield/War Mace, he wipes out a wall spawn with one run though his AOEs - Build up, Shield charge, Whirling mace, Crowd control and Shatter = everything dead
I'm with Star_Seer on this. Shield/War Mace. Shield Charge has to be one of the best (tank) attacks in the game - Build Up + Shield Charge = entire spawn of whites dead with my slotting. I'm a perfectly good tank, but fully able to solo as a scranker as well.
Take Rage out of SS and Build up out of Fire Melee and you can soon realize that Fire Melee offers a much decent attack chain. Rage buffs a subpar attack chain whilst Build Up provides peaks and troughs, and what peaks. Its doubtful SS competes in the AoE dept, end of, but even with stacking rage you have to ask yourself what mobs resist, what do AVs resist? Are they going to resist Fire more or Smash more?
He will honor his words; he will definitely carry out his actions. What he promises he will fulfill. He does not care about his bodily self, putting his life and death aside to come forward for another's troubled besiegement. He does not boast about his ability, or shamelessly extol his own virtues. - Sima Qian.
I would say SS for the SMASH. I have a WP/SS tank, a SD/SS tank and a SS/SD brute. so I may be some what biased.
Also foot stomp has a larger radius than fire sword circle.
But then fire has the end in been less resisted.
But hey who can't love the KOB animation.
Thanks for all the info guys.
I made one of each and then stuck with the SD/SS - the concept came out better.
Concept trumps all 
L50s: Tanks: Cryofission - Ice/EM - Dr Celsius - Fire/Ice - Saint George - SD/SS | Controllers: Psichosis - Ill/Kin - Major Chaos - Ill/Stm | Scrappers - Neutron Crusader - DM/SR
Currently Levelling: Angelic Blade - BS/WP Scrapper | Seeds of Destruction - Plant/Kin Controller
Hi all.
I'm looking to make a high damage Tanker and was left tossing up between these two - Shield/SS or Shield/FM.
How much difference is there in damage and how much damage mitigation has SS got in it - I know FM hasn't got any?
I'm just after opinions and general thoughts really.
Thanks
L50s: Tanks: Cryofission - Ice/EM - Dr Celsius - Fire/Ice - Saint George - SD/SS | Controllers: Psichosis - Ill/Kin - Major Chaos - Ill/Stm | Scrappers - Neutron Crusader - DM/SR
Currently Levelling: Angelic Blade - BS/WP Scrapper | Seeds of Destruction - Plant/Kin Controller