As I said before in private, the 18" will outperform the 14.5" almost always. A 24" would outperform an 18" in almost any situation.
Aperture always rules as long as the optics are of good quality. Yes, there are those that say that a smaller scope will perform better under certain atmospheric conditions.
But, aperture also means size, weight and height. This means that you must have a larger vehicle to transport it and that you will have to lift heavier components for assembly. This may require you to have help for assembly and disassembly. It also will mean that you may have to use a ladder to view.
You must think about these things and be sure that this is what you are willing to do. If you find that it is too much, be aware that you will likely loose money when you sell it. Note the ads for the larger scopes and the prices for them almost new! It really can be a costly mistake.
If you really want a larger scope. Then I suggest that you ask your friend let you assemble the scope and put it away after a full night's viewing. This will allow you to experience everything about using the scope. It is always harder to put it away than to put it together! 8)
I love to look through a large scope. It is very satisfying and a pleasure. But I have tried to put together a 20" by myself and was not able to do it without straining my back. This is the reason for my owning the 14.5". If I could handle an 18" I would probably own one.
Hope you find what you are looking for in the end.
I keep hearing past conversations with people about aperture fever and climbing up and then finally done the ladder until you find your comfort zone. This is reality and you see it all the time. No one in our club takes out there scopes over 16" with and frequency. I have only seen one of the two 17.5" scopes in the last three years and it only two time. The 20.3" scope has been out maybe 3-4 times in these three years. Even the 16" is many times not there and is being left at home for a 14.5" scope that he owns.
Not trying to make any waves or cause any argument, just trying to show both sides of the story.