A good rule of thumb is that the more expensive one of a pair of devices made by the same maker is better. Comparing prices in the same shop gives the answer then.
But that's not the whole story. Usually when a new model appears, the old models are reduced in price. That doesn't necessarily mean that the new one is better. But the difference might be rather small, and you might prefer to buy the good enough older one.
Things to do:
1. Read the reviews on the web.
2. Read the comparisons from https://www.google.coml/search?q=one+plus+2+vesrus+one+plus+x
3. Ask your friends that have one of these models.
I'm thinking of buying a new phone and I'm really interested in buying a Oneplus model
I have two choices in hand
Oneplus 2 OR oneplus X
which is better?