I’d go for the 127mm

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#580257
Paul Leyland
Participant

Personally I’d go for the aperture if it’s within your budgetary and mechanical limitations.  Most of the time the image will be seeing limited but the resolution of the larger will be better than the smaller in the brief intervals when the atmosphere in front of your telescope is steady.  The extra light grasp will be invaluable if your tastes change and deep sky becomes more important, or if you branch out into fields such as VS and cometary observing.

As for focal length, note that 13*102 = 1326 and 9.5*127 = 1206.5 so the longer focal length is only 10% greater than the shorter.

Added in edit: (127/102)^2 = 1.55, so the light grasp is 55% greater, or roughly half a magnitude.