I am going to play Devil's Advocate here. I agree that slip fittings allow in some cases for more compact connections and this can sometimes be an advantage. But, personally, I will always choose threaded connections if possible, and will even allow extra space for their use, simply because they are not permanent and allow relatively easy modification or disassembly if future changes are needed or desired. Slip fittings require gluing; once together...they are together for good. Future alterations or mods then become far more difficult, and involve much cutting, cursing and effort.
Another minor point is that threaded connections can be assembled and put into service immediately, with no waiting for solvent-cemented slip connections to cure and become safe.
I of course agree that, ideally, one plans out the entire plumbing project ahead of time, ensuring that everything is purchased and ready to go. I follow this procedure religiously; and, in over 5 decades of fishkeeping DIY...it has never once worked out that way! There's always some forgotten detail, some unforeseen glitch, some accidentally broken part, some wrong-sized piece...
something...that demands another trip to the hardware store or, worse yet, another mail-order delay, before the project can "get wet".
Nowadays, if I need an XYZ widget...I will buy three of them. Use one, drop the other two into the parts/junk bin, in hopes that it will come in handy at some point in future. The first time that happens...the first time that you plunge your arm into the bin and pull out the magic gizmo which will allow you to connect
this to
that and thus save the day...you will be chortling and grinning from ear to ear.
0h, and regarding the worry about threaded couplings leaking eventually? I use teflon tape on all threads, tighten snugly but not excessively...and can't remember the last time I had such joints leak. I have at least a half-dozen threaded bulkheads right now that have been in service for a decade, haven't been touched or serviced...and don't leak. I'm pretty sure that I had others that were considerably older in my previous house. How long do you need them to last?
If one did begin to drip...I'd unscrew it, clean the threads carefully, reapplly teflon tape, screw it back together...and sit back for another decade.