Mic for recording sound demos ?

I believe that you are talking about recording sound coming out from the speakers of your stereo system. I have seen a YouTube video about such recording but from one speaker only with a blue yeti and the result is quite impressive. See  https://www.youtube.com/watch?v=or-cwTKmk_8   For stereo recording, I think that you either have to use 2 microphones, one for each speaker, or if you use the stereo function of the blue yeti, you may have to put the speakers close to the microphone. If the microphone(s) is too far away from the speaker(s), the room acoustics will play a great part in the sound received by the microphone(s). 

Yes I actually did this and was really impressed with how good it sounded! The stereo mode of yeti.

The Yeti is easily good enough to do the job and will outperform the dynamics especially at high frequencies... the built-in ADC pretty good as well. I suggest you check you're getting the most out of the Yeti by checking your gain settings to ensure you've got a healthy input level and the signal is not clipping. Also check the recording parameters - bit depth (ideally 24) and sample rate (at least 44.1kHz).

By the way, the Yeti has three capsules so is in effect three microphones... these get mixed together to produce the different frequency response patterns. If you don't have a whole lot of experience then a single stereo mic like the Yeti will help you avoid the hassle of trying to phase match a spaced pair of mics (look up xy configuration if you're going to use two mics).