All 3 emulators will offer the same amount of customization. You won't get source code with sphere as you would with RunUO and UOX3, but if you cannot program then it wouldn't do you any good anyways.
There's nothing that cannot be scripted in any of the 3.
Sphere:
PROS - the learning curve is very small, it's easy and you can dive in and start editing. The more complex your scripts, the more you'll have to learn. There's a huge community, and if you don't come across as a total loser looking for handouts, there's plenty of people willing to help you with problems. There's oodles of publically downloadable scripts for sphere. There's many many programs compatible with it for gm tools, mul editing, etc. The rumours about it being limited or weaker than the others are simply not true, it's a strong scripting language and can do anything you want, assuming your goal for UO isn't to script instant messengers that'll ICQ people...
CONS - once you've mastered the language, it's useless to you for anything other than sphere. while it will provide a basis for you of proper if/else structures, the fundamentals of programming, etc.... it's still just a scripting language usable only in one program.
UOX3:
PROS - You already knew js, it's scripted in js, it'll be something akin to re-learning how to swim or ride a bike
It's open source, so if you ever did get the urge to program, you can edit or fix things. Xuri works on it, if something's broke you can just harrass him here
You'll have a solid mastery of javascript once you use it enough, this can be used elsewhere in life.
CONS - The community is smaller, at least it was.. I haven't looked lately
There's not as much freely available support or pre-made scripts for it. It's a steeper learning curve than sphere, but only if you cannot remember all the javascript you did know.
RUNUO:
I'll let someone else give it, I'd be too biased with the CONS and not the PROS