Are you asking for evidence of donation or of the ACLU doing far more good than the NRA? Both seem to be odd questions.
The NRA claims that protecting gun ownership protects civil rights by empowering the individual to defend themselves against the government (we'll ignore, for a moment that nothing could be further from the truth, and everyone in this nation, armed or not is a heartbeat away from a smart bomb at their breakfast table, or that you can be financially and socially ruined without ever having the opportunity to shoot back). Let's take the NRA's claim at face value and assume that they are 100% correct.
They still only defend the status quo. Having a gun doesn't undo the erosion of rights due to the corrosive influence of the re-election cycle in Washington. The ACLU seeks to actively move the line of civil rights back to where it started, and hopefully even a bit further through the courts and activism.
Now, the ACLU and the NRA happen to disagree over the interpretation of the 2nd amendment (FWIW, I think that was the stupidest call the ACLU ever made) but even when they disagree they're still nominally working toward the same goal (the ACLU isn't trying to prop up the gun industry, but I'm talking about implied goals, here), so it's pretty easy to judge which of them objectively makes the most progress...