Onfi and Vimpat are the two medications that my daughter is on, so hopefully my comments will be helpful to you.
Onfi never worked well for her. I don't think that it was due to tolerance - it is just not a drug that was able to control her seizures well. She did start off at 20mg/day, and within a few months was up to 55mg/day (pretty much the maximum dose), but her seizures were only partially controlled. We also never really saw any impact on her anxiety, even at very high doses of Onfi, but your mileage may vary! On the plus side (if there are any pluses to a medication that is not controlling seizures!) at the lower doses of 20-30 mg/day she didn't really have any side effects; however, once she got over 40mg/day it seemed to be pretty sedating and she was tired all the time.
She started Vimpat at the end of November and since then she has had zero tonic-clonic seizures! She does continue to have simple partials, but now that she is at the maximum dose of Vimpat (400mg/day) they are happening much less frequently. When she first started on Vimpat she had a few weeks of exhaustion and very bad headaches, but they disappeared after about 3 weeks. She had no issues whatsover with a recent increase from 300mg to 400mg/day.
We are hoping to eventually get her off of the Onfi, and have been reducing it very very slowly - she is down to 35mg/day Onfi in addition to 400mg/day Vimpat. You need to reduce Onfi slowly because there is a danger of withdrawal seizures if you go too quickly. I do feel that she gets a bit anxious with each reduction, but it is hard to say whether that's a physiological reaction or a psychological one since she is so concerned about having seizures.
So for my daughter, Vimpat has been a much better medication than Onfi - but of course your reaction may be different! One drawback to both medications is the cost. Neither of them have generic options, and with our insurance plan our copays are ridiculous ($250/month for each!). Hopefully your insurance is better.