What would be more DPS for my rogue? Upgrading my haste item or offhand?
 

I currently have an RBG for my haste item(with epic of course) and a winter's fury for my offhand. Would I see more DPS from upgrading the winters fury to an Exquisite Velium Spear or upgrading my haste item for more backstabs to an RBB?

Both either way will give you that extra Dps boost.
I would go RBB first then wep upgrade after that.

4% haste upgrade... meh. Get the shiny 12 20 weapon imo if you can stand that spear look for probably forever. I can't.

I'd go with the offhand upgrade. Your backstab refresh timer will drop by about .24s~ (7.87s vs 7.63) when you're unhasted. The benefit of upgrading your haste item is mitigated by haste, because even with celerity (shaman 50% haste) the value of that 4% haste is essentially halved (5.52s vs 5.4s).

Ideally you'd get a better haste item in a different slot so you can use a turtle belt (or dain belt) for the aura of battle.

What about if say I skipped the RBB and kept saving for a CoF? Would I see a better DPS increase in getting the CoF vs my RBG instead of upgrading to the spear over a winters fury?

If you want to calculate how much better a CoF is going to be then take (10/1.36) which means backstab cooldown will be 7.35 seconds compared to an RBB would be 7.63 seconds. Currently with your RBG your bs cool down timer is 7.87 seconds. As you can see even a CoF isn't going to make that big of a difference. You'd be better going off for the exq. Velium spear imo

Like sdp00888 said, upgrading your haste is minimal gain, I'd go with the offhand. The benefits of a haste upgrade get mitigated by having a buff haste (which you'll have in nearly all scenarios), whereas the benefits of a weapon upgrade are magnified by having haste buffs, as well as most likely providing more DPS in any scenario.

any direction u go will work. eventually u will upgrade both. I would upgrade haste because selling your rbg for RBB will be a cheaper upgrade.
upgrading offhand will only increase your offhand dmg.
upgrading haste will increase your backstab, ragebringer and offhand dmg.

I'm pretty sure cof is better than upgrading offhand. 3.5% offhand upgrade vs 9% bs cooldown. I'd even guess than RBB is slightly better as well. Every rogues damage splits will be different, but mine is roughly 40% bs / 40% mh/ 20% oh

Upgrading his haste won't affect auto attack since he uses ragebringer

Upgrading to CoF is not even a 9% cooldown increase unless he's unbuffed. It's only 6% with just Speed of Shissar.