Translation:Do not take this, take something else.
Bierz => brać (to take) (continuous action).
Weź => wziąć (to take) (single action).
There is no ' nie weź' - as an imperative.
Also "Nie bierz tego, bierz coś innego." is very coloquial, and I don't think entirely correct, maybe is some contexts.
You cannot switch between single and continuous action freely. And it could be hard to know when to use one or the other, because in many cases they do not translate literally from English.
I think there is only one way to use negative imperative - imperfective form, simply because for almost all verbs perfective form doesn't exist :).
Obvious exception is Idź (go, perfective) VS chodź (go, imperfective).
Nie idź tam - Do not go there (Do not end your journey (as moving from A to B) there)
Nie chodź tam -Do not go there (Do not make a habit of going there).
Also chodź can be used as an imperative of come - Come here! - Chodź tu!
But these are movement verbs - they are a challenge in every language, so good luck!
"Positive commands usually occur in the perfective aspect, while negative commands occur in the imperfective."
Swan, Oscar (2008-10-12). Polish Verbs Essentials of Grammar, Second Edition (Verbs and Essentials of Grammar Series) (Kindle Locations 1305-1306). McGraw-Hill Education. Kindle Edition.
HOW TO REMEMBER WHICH is WHICH
For English speakers there's an old-fashioned or colloquial form of negative command - "Don't be standing there!" (roughly equivalent to "Don't stand there") but its expressed in English in a continuous tense = imperfective.
So what will help me remember this is to translate the sentence into "Don't be taking this, take something else instead."
The above construction is very popular in Irish English. If you know this construction in your English then its a good way to remember that the imperfective is used in negative commands.
On the other hand switching the aspects to give: "Don't take this, be taking something else instead." is not seen / used in English.