Going from 10" to 15" is a substantial jump, about 0.9 magnitude. Jumps that big tend to have a "wow factor." 15 to 18" is an additional 0.4 magnitude, but 1.3 magnitude over the 10". Since you are keeping the 10" the question is more of where the extra aperture becomes a hassle/problem to transport and use. That depends on you and your situation.
Consider how you would use the new scope, how you will transport it/how it will fit when loaded, what additional stuff you will need with it (e.g. ladders or observing chairs), who will be along with you for the trip, and the associated stuff for camping, etc. Plan for a scope that you can reliably load/unload without any help. This should help you define the maximum practical size for your needs. And consider whether you will still be able to handle the same scope in 5 to 10 years.
There is some advantage to maximizing the useful aperture while one is young--better eyes and better fitness for using the scope, rather than realizing one can no longer go bigger a decade or two later. However, if you undershoot a little on aperture, it is no great loss either. You might find some things are incrementally out of reach, but should have a more manageable scope as compensation. It will probably not be enough of a difference that you will actually regret undershooting and end up switching to the next size up. On the other hand, if you overshoot by an increment it could very well limit the usefulness of the scope to you in a significant way.
This.. Particularly the last line:
"On the other hand, if you overshoot by an increment it could very well limit the usefulness of the scope to you in a significant way."
Most big scopes don't get used a great deal. Slightly smaller but more often is preferable to larger but a just bit too big.
What do you enjoy observing?
What are you hoping to see in a larger scope ?
How tall are you?
How often do you observe?
I often see people recommend the biggest scope you can manage, the biggest scope you can afford.. And yet I see numerous people buy a large scope , have it for a while but not use it much and then part ways with it. A big scope that gets used 50 to 100 nights a year is not all that common.
Between 15 inch and 18 inches is 16 inches and there's a lot to be said for a 16 inch. It's goes a full magnitude deeper than a 10 inch, is only a quarter magnitude down from an 18 inch but can be more easily managed than an 18 inch and more moderate focal ratios are still flat footed.