Lash stylist usually use 2 types of tweezers: so-called straight and curved ones, although there are plenty of different shapes of tweezers for both isolation and application. I would not say that one shape can be better than the other, the choice of the proper shape depends on fan forming technique and just personal preferences.
When choosing the tweezers pay attention not only on the shape, but the length of the grip and how it actually feels laying in your hand. Find the «grabbing zone» and try if it works for you.