I want to do a microneedling RF. How to choose the machine? Endymed or Vivace? Which is better and what are the differences?

There are new microneedling RF devices coming on the market all the time.

The choice may rely on both what you want to achieve and what device your physician has available. These devices can get quite expensive and many small suburban dermatology practices aren't in a position to invest in the topmost technology that can cost hundreds of thousands of dollars for a single machine.

Needle penetration depth is a key factor. If you're looking for superficial improvements, there are a wider number of choices.
Since some of the RF systems are only fairly superficial in their treatment capability, it's wise to do a bit of research.

Look into the Lutronic Genius, Candela Profound for serious capabilities. The Genius device is the only one to date that has "real time" feedback for complex customized RF energy delivery. But typically requires multiple sessions. The Profound is a single session procedure that's quite adept at firming the lower face.

Again, the problem you want to treat will have a lot of bearing on which device you would get desired results from. If you're confident in your physician's skill, let him or her be your guide.