Hi,

I just got a Sony pxw-z90 and it has an internal microphone. I also had a MC2500 camera that came with a stereo shotgun, ECM-PS1, I plan on hooking it up to my z90

I'm wondering if this mic will sound better and if so, why?

Those mics are really just cardioids in a small shotgun-esque looking package - but are an electret at the end of a tube - so pickup from behind is screened off, which produces a strange result - a pretty wide cardioid with little rear pickup. Personally, the one in the camera will probably sound nicer, but will pick up the sound to the sides.

The Sony PWX-Z90 gets good reviews and you need to decide what type of audio you’re going to record when you put a new mic with it. Usually it’s good clear voice work, and you don’t need a stereo shotgun for that, but with careful positioning it should work, if you’re careful mixing the left and right channels in post.

For voice work, a good directional mono shotgun will beat the Z90 Omni directional stereo camera mic., especially in noisy environments.

The Z90 has professional type, 2x XLR-type 3-pin (female) line/mic/mic inputs with +48 V selectable. The prosumer ECM-PS1 mic. has a 3.5mm output plug, so they’re not compatible.
Cheers.

Depends I what you intend to film. Generally internal mics serve the purpose of backup, ambient, sync source or down and dirty work where you don't have time or access to a better source. For interviews you're better off with a lav or shotgun off camera on a boom, for event work you'll often be mixing the internal with a board feed to give a more balanced sound. The downside of internal mics is that they can pick up camera noise or handling noise if you touch the camera while recording. The other downside is often the camera is too far away from the subject to get optimal audio. People general don't want a camera a couple feet in front of their face. You'll also have to use widest angle to fit them in the frame which distorts their face up close like that. That's why it's best to use an off camera mic.

Never heard of that other mic. I would stand 3 ft away from the camera and film yourself talking with the camera internal mic and that other mic. Then see if you can hear a difference. You find real world results based on how you'll use it is far more important than any spec or review you can look up.

Bottom line is it depends on what you are recording, the venue, and the positioning of the microphones. The physics of audio recording are that in general for voice and point sources the microphone should be very near the source/talent (i.e., within a few feet). A shotgun dose not "reach" further, it just reduces sounds coming from the sides and rear according to its pattern, and that could add unwanted aspects to the sound in some cases.

As suggested by Pete above, run tests in an environment that simulates your intended shoots to decide what works for you. And check to be sure you have any needed adapters for the external mics you plan to use.