I recently bought a Marshall 1960A Lead quadbox for my 5150 head. Which setting in the ohms do you recommend i use?

Ummmm! Use the right one! You can fuck the amp up beyond all belief if you don't use the right setting. Whatever the Ohm is on that "single" cab is the Ohm setting to use on the amp. That all changes if you are using other cabs at the same time though. Never turn the amp on without checking to see that the cab is pluged up and the Ohms are correct.

__________________

ok, i can have either 16 or 8. Which one would be better?

The cab has a switch or are you talking about the amp? We got to be clear about this.

__________________

It's best to run matched ohm ratings on the amp and cabinet. But it is also safe to mismatch one step. Meaning that if you have a cabinet that is 16 ohms and your head can do either 8 and 16 you can use either safely. Do not go in the other direction though and run a higher impedence rated amp into a smaller impedence rated cabinet, especially if you are running tubes. Mismatching your amp a step down in rating will give a bit more saturation/compression to your tone. I don't think that most will notice this at all if you are running a lot of preamp gain though. You will notice this more when you are getting poweramp clipping.

As a disclaimer, if you do not feel comfortable doing this then don't.

I run NMV Marshalls and have run my amps on the 8ohm tap into Marshall 16 ohm cabinets for years and have not lost a tube or transformer from doing this.

EDIT: One more thing. It is not advisable to run two mismatched cabinets to a head. Such as a 8ohm cab AND a 16 ohm cab on the same head. You would have to use the 4ohm tap on the head if it has one as you will be pushing roughly 5.8 ohms through the circuit which WILL shorten the life of your transformers.