How to compare two CSPRNGs? Let's say I am given to CSPRNGs: RNG1 and RNG2.
I want to select the best one of the two for cryptography.
How should I do it?
Any of the two RNGs can be initialized with a secret key during
tests and any number of bytes can be generated from each RNG.
Are there statistical test suites already out there for this purpose?
The best one is the most reputable one. That is, the one which had been widely used, studied, and standardized.
If both fall into that class, choose the one which uses more secure or larger-state primitives under the hood.
Example: when comparing the NIST-standard Hash-DRBG using SHA-256 versus HMAC-DRBG with SHA-1, I would personally choose the first due to larger state and security margin of SHA-256.