Hi, I would seriously consider the 174mm as it has a global shutter as opposed to a rolling shutter ( so does the 178 )and has a higher frame rate also ( 128fps ) at full frame resolution. I also have a 178mm and only use it if the seeing is very steady which is hardly ever. I also image the moon as my primary target and have an 18" newtonian at f/4.5 and mostly shoot at f/14.
In any event whichever you choose I would get the Astronomik 742 IR Pass filter ( 1 1/4" ) which blocks all visible light and passes infrared which has longer wavelengths and blocks the shorter waves which scatter more and kills detail. As soon as I started using the filter I got a very high percent of more detailed images compared to so-so images that were almost there but not quite. I noticed a dramatic improvement on the monitor images right away and the minute detail was easily apparent even in poorer seeing. It will give a slightly lower frame rate but the quality improvement more than compensates. I'm waiting for the Astronomik 642 IR Pass filter which doesn't go as far into the infrared longer end and is supposed to give the same exposure as without it.
Where I live in Buffalo NY the seeing is usually very jumpy almost all the time and the IR Pass filter
works wonders and greatly improves the minute detail when without it I would get nothing usable.
As you can tell I can't praise this filter enough as it was a turning point for me.
