Juvederm Ultra is slightly smoother to inject and slightly less likely to form a bump, compared to Restylane. For "unforgiving" areas, like the lips and tear troughs, I think Juvederm is a better product because it has a smoother consistency. For "forgiving" areas, like the nasolabial folds, marionette lines, frown lines, etc..., I think Juvederm or Restylane are fairly comparable, lasting about 6-8 months on average.
If cost is a consideration (and both cost about the same per syringe), I often recommend Restylane. The reason is that Juvederm has 0.8cc in a syringe, while Restylane has 1.2cc in a syringe. Therefore, you have more material to use with one syringe of Restylane, enabling more folds and wrinkles to be improved. Unfortunately, Juvederm doesn't come in any other size, while Restylane also has a 0.4cc syringe (which is cheaper because it is 1/3 the amount of a 1.2cc syringe).
Similary, if a person needs more than one syringe of product for correction, it takes 3 syringes of Juvederm (2.4cc) to equal the amount of product in 2 syringes of Restylane (total 2.4cc). This means you have to spend money to purchase an extra syringe of Juvederm.
I think the decision of which to use between these two fillers should be a mutual one between you and your treating physician. Treatment goals, area to be treated, cost, and amount of product required are all key factors.