SSG = 3% total avoidance (1% miss, dodge, parry), +2% stam
SS = 4% parry
A lot of tanks will take SSG for the health bump, not because they particularly need the defense (I do because I'm a Blood tank and I really do like more health). You are right though, in that SS will give you twice as much RS/SoB triggering chances, and for that it is a fine choice. For posterity, note that neither rune will be affected by, or affect your diminishing returns on avoidance stats.
If you're having trouble with threat, by looking at your gear (which I notice is setup to stamina at any expense), I'd wager that you are just pitting your threat against better geared dps, am I right?
By your equipment I'd guess you'd put out fine threat against people up through Ulduar gear, but if you're going against people in healthy amounts of t9 you may have to work hard for it, and your weapon will contribute to that a fair amount (219 ilvl, 212 dps will hurt your OB, RS, BS, and FS a bit compared to higher geared dpsers). Your avoidance is decent, as is your health.
What level of content are you tanking, and who are you tanking for? I think your methodology sounds just fine.