Each product has its pros and cons. However, my personal experience has caused me to use Juvederm much more than Restylane.
Similarities:
Both products are safe and effective.
They cost about the same.
They are both made of hyaluronic acid, which is a normal component of your skin's matrix. That is why, unlike some other collagen based products, allergy testing is not needed. Because of this you can be treated on the same day as your consultation.
Both products tend to be smooth and plane out after injection.
Key Differences:
Although Restylane typically lasts 6-9 months, Juvederm often lasts 9-12 months and has FDA labeling saying that it "may last up to 12 months."
Microscopically, Juvederm is a smoother compound than Restylane, although I am not sure that it behaves much differently in patients.
Juvederm has a higher concentration of the active product, hyaluronic acid.
A treatment session with Juvederm uses a complete syringe (it can't be shared or stored for later use). Restylane is offered in both 0.4cc and 1.0cc sizes, so you have the ability to purchase a smaller syringe at a lower cost if you have a low volume issue requiring correction.
Also, Restylane syringes typically have 1.2 or 1.3cc while Juvederm has 1.0 or 1.1cc. So, you get more physical product with Restylane than Juvederm.
The reason that I use Juvederm more is that I feel it does settle in smoother. My patients agree on this fact.
I also believe Juvederm lasts a little bit longer than Restylane.
Finally, the design of the syringe makes it easier to inject, increasing the precision of the injections.
