Stu,
If you liked the 5" APO over the 18" Obsession 1 of 2 conditions must exist
1) the Obsession has a pretty average mirror in it.
2) you have far different observing preferences to me
The visible detail and resolving power of a 5" telescope is not even in the ball park with an 18" telescope. Yup it will give nicer star images but thats about it.
CS-John B