In terms of light grasp, I think one would want to go at least 1 magnitude deeper - a total brightness ratio of 2.512. Think about the difference in a sky with a naked eye limiting magnitude of 5 compared to one with a limiting magnitude of 6. Or a limiting magnitude of 6 compared to one with a limit of 7. To me the difference in seeing the Milky Way, for example, is significant in each case.
So a one magnitude increase requires an aperture increase of SQRT(2.512) = 1.585. I have a 6" refractor and a 10" reflector. 6X1.585 = 9.5. So the reflector goes about I magnitude deeper than the refractor.
In terms of resolution of fine detail, I think Edggie has said it best in other topics. To see more detail one needs aperture. A 160 mm scope will be able to show a larger image than a 130 mm with the same brightness by a factor of (160/130)^2 = 1.515. In other words, you can observe extended objects with the larger scope at the same brightness level, but they will appear 50% larger. A 140 mm will allow (140/130)^2 = 1.16, i.e., a 16% larger image at the same brightness level.
Based on the above, which upgrade will provide the best value, keeping in mind that the upgrade will also require a bigger mount? My personal formula for deciding on scope size: 1) Determine the largest scope(s) I can comfortably handle. 2) Get the best I can afford or wish to spend, as opposed to THE best.