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Aperture rules.
Ultimately, we all settle for a scope that we can manage which is smaller that we would like. My rule of thumb when shopping around is to find the size of scope that scares you at the thought of moving and buy the next one down. That's why I settled on a 20" f5.
I have seen the Skywatcher 20" f4 at Sirius Optics and it looks quite small by comparison. The 18" must be positively diddy! Although there are a lot of bits to build it, it does make it easy to transport and it seemed to be pretty well made. MUCH better built than the smaller chipboard mounted siblings.
I would warn you though, if you go down the f4 route you will almost certainly need a Paracorr of some description and they are not cheap. The coma will be very evident and will need correcting to fully appreciate the instrument.
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