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I have been observing for 40 years this year and I've never been able to see anything through a 5" telescope ( and I own a very high quality APO) that gets me remotely as excited as what I can see through a quality 10" plus telescope.
An 18" has sufficient light and resolution to keep a dedicated observer occupied for life. And the quality of the views will not reveal as easily as a small scope with relatively poor resolving power, that gives images that appear sharp almost always but offer little detail. One has to really have a grip on finding a suitable site free from localised seeing issues- understand cooling and collimation and understand atmospheric seeing . When everything comes together views can be revealed that will be etched on your mind for life.
A quality 5" scope is not able to transcend the laws of physics no matter what type. My general rule of thumb is that refractor can give views similar to a high quality Newtonian around 20% larger . But that puts your available light gathering at around 8" at best.
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