The pixel scale (with 3.75 µ sized pixels on an ASI120) is really not too bad for your long focal length; except for the ASI174, you can't do much better until ZWO starts allow 2x2 binning for guiding.
However, the use of a color camera (not just the ASI120) will be detrimental to sensitivity. Each pixel will receive only about 1/3 of the photons from what an equivalent mono camera takes in. I.e., you will likely pick up perhaps about 50% of usable guide stars that the equivalent ASI120MM can pick up. This is where the ASI120MC pick is arguably the worst choice out of all the available guide cameras that you can use with an ASIAIR.
The read noise of the ASI120 is also on the high side compared to the better cameras. This will not hurt single star guiding much, since you only need one star to guide, although it can drop stars more frequently when poor seeing really scintillates the star, especially if the star intensity is dim to start with. It is hard to find bright stars in the FOV of a long focal length OAG.
If you plan on using multi-star guiding, where all chosen stars need be above the noise, while the brightest one remains unsaturated, then this camera will further cripple guiding.
The dimensions of the sensor will limit the number of stars that you can use (both for single star guiding, and especially for multi-star guiding).
But since the ASI174MM is not cheap, I would recommend at least trying the ASI120MC first, if you already have the latter camera on hand, rather than throwing more money away. A ASI290MM has better dynamic range (you can use dimmer stars) but has comparable FOV as your ASI120MC.
Autoguiding is a bunch of compromises, including money. It is best that you come up with something for your own situation since it will be different from someone else's. However, if money is no object, I would throw my hat in the ASI174MM direction when it comes to using an OAG with a long focal length.
I have tried using an ASI178MM for guiding, but the frame rate is too low since the ASIAIR does not have a ROI adjustment for guiding. When using an ASIAIR, you actually don't have many cameras to choose from, which makes the choice mercifully simpler.
Chen
