I don't think twisting will be a problem since I have a traveler bar across the back
Your traveller is only effective in helping control twist if the boom is still above it. Once you're going downwind and the sheet's let off there's very little downward component to the mainsheet pull. This is where a vang is helpful, it can be used to limit boom lift, provide leech tension and limit/manage twist.
If I had to pick just one, it would be the vang. A powerful easy-to-use vang is like having an infinitely wide traveler.
As an example, look at a competitive Laser dinghy.. a pretty mediocre traveler arrangement, but you'll see a high power vang.