You'd probably find the 18" UC a lot easier to transport and setup than the SDM 16, and it's pretty easy to see the difference in the eyepiece between an 18 and a 16 side by side. You can be pretty confident the optics in either will be good, both SDM and Obsession use reputable mirror makers. The only downside to the UCs is the potential for the collimation to drift - but I haven't heard anyone on the Obsession group complain about it for a long time now, so I guess it's been sorted.
Either way - nice problem to have!
cheers,
Andrew.
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