Defence is when you avoid the damage by them missing. Resistance is when the damage taken gets reduced.
MA/SR is good because theres no drawing time when you practise brawler (mes protection which causes you to put your sword away) and does slighty more damage as well as having stuns for damage mitergation.
Kat/SR is good because you get divine avalanche which stacks with your melee defence making you pretty much unhittable in melee. Katana also has more AoEs than MA but is slighty more end heavy.
Either combo is great just a question of personally preference 
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
I took a MA/SR to 50 and really enjoyed it, still one of my fave characters.
A few things about MA...
1) You have no redraw animations, so you'll get great mileage out of pool powers like Air Superiority. It's much better than Thunder Kick.
2) You do have some great stuns and knockback, which some people say makes MA a "scraptroller." Great damage mitigation for a Scrapper. Just watch out for Crane Kick if you team, try to save it for a finishing move so you don't keep knocking enemies away from the tank.
3) Endurance was very tight for me until Stamina. At endgame you're running at least three SR toggles and constantly attacking on top of that. Be prepared to slot for Endurance Reduction if you have trouble. I put one EndRed in each of my attacks and one in each of my toggles. (you'll find Stamina in the Fitness pool. When three-slotted I find it solves a lot of my problems with End)
Katana is also a great set, though I have much less experience of that and only as a Kat/Regen. It is true though that with the defence bonuses it gives it's probably got a better synergy with SR than MA does. But MA/SR looks cooler IMO
And yeah, Defence is about avoiding damage completely, Resistance is about taking the hit but reducing it. This does mean that a SR can be quickly killed by a run of bad luck or an enemy with a significant accuracy boost. Generally it's pretty survivable though.
"The Hamidon is a what what of what?" - Brian the mission guy.
True. I've seen an SR scrapper stand in the middle of a mob, without elude, and manage to only get hit 2 or 3 times. I've also seen the same SR scrapper jump into a mob on the same mission, and by sheer bad run of numbers, faceplant within 10 seconds.
Defiant 50's
Many and varied!
@Miss Chief
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True. I've seen an SR scrapper stand in the middle of a mob, without elude, and manage to only get hit 2 or 3 times. I've also seen the same SR scrapper jump into a mob on the same mission, and by sheer bad run of numbers, faceplant within 10 seconds.
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the bad thing about defence sets 
The fun thing about defence sets. SR is like walking a tightrope sometimes. The potential for sudden disaster keeps me on my toes.
"The Hamidon is a what what of what?" - Brian the mission guy.
SR is a great secondary, got a spines/sr srapper and he's as mad as a bag of cats.
As the others have mentioned it can be hit and miss sometimes whether you sail through or faceplant in a mission, so taking aid self (medicine pool) really does help your survivability.
Yes aid self is a must as SR is the only Scrapper secondary without a heal.
Hey guys. I'm thinking of making a Scrapper, but for concept reasons his secondary is going to be Reflexes. Just wondered whether Katana or Martial Arts would be better for his primary. Any advice?
Oh yeah, noob question, what's the difference between Defence and Resist Damage?