For gaming, most definitely the one 34" screen. Especially for 1st and 3rd person games.

For all other things, dual screen is probably better. Most content on Netflix is 16:9 (except movies), so playing it on a 27" monitor and then you'll still be able to do other things on the other monitor.

The important thing is you need to know which is more important for you.

My suggestion is if gaming is the thing you do most on your PC, then get the 34" (make sure that the games you play also support it - e.g. Overwatch has poor support for 21:9). If later ,you still feel you need and extra screen, then maybe you can save some money and get the 27" to put it at the side (or maybe a different size depending on your needs).

But if you mainly use your PC for things other than gaming (especially for work), then 2x 27" would be better IMHO.