Xor or single ended?

That should read "rca" and "xlr"!!

A phono cartridge is actually an inherently balanced design - it has  a separate hot for left and right, separate grounds for each, and a shield. That makes phono connections ideal for balanced phono preamps, because a truly balanced (differential) phono preamp will have higher gain and lower noise than a single-ended preamp, all other things being equal. Please note that you can have a balanced phono connection using RCA connectors; it's the circuit, not the connectors, that make it balanced.

Thank you Cleeds for a clear and informative answer!

Jake

Dear azjake: cleeds gave you a good answer and the main subject there is:

"""  a truly balanced (differential) phono preamp """

if your phonopreamp is not exactly designed like that hen it does not mattes about how you connect it. Many phono stages comes with rca and xlr input connectors but in reality are not fully differential, so you can get the advantage to lower noise/distortions.
You have to ask on that issue to your phonopreamp manufacturer.

Regards and enjoy the music,
R

cleeds is basically correct but not entirely correct.  The coil in the cartridge has two free ends; I think we can easily agree on that.  When you use the cartridge in single-ended mode, one end is connected to the hot pin or the hot female input of the RCA connection, and the other is connected to audio ground via the collar of the male RCA jack or the sleeve on an RCA input jack.  However, when you hook up a cartridge in balanced mode, there is no audio ground connection; each end of the cartridge is attached to one of 3 pins in the XLR, typically pins 2 and 3.  Both sides see signal. Nominally, pin2 handles the positive phase, and pin3 handles the negative phase.  Ground floats. The third pin of the XLR (pin1, typically) carries ground from the tonearm, turntable, and/or the shield around the phono cable to the chassis of the phono preamplifier. Balanced operation helps to cancel noise and in my experience nearly always reduces or eliminates grounding issues.

But like Raul wrote, if your phono stage is not also fully balanced, there is little to be gained by using a balanced connection from the cartridge to the phono stage.  Some SE phono stages use a transformer input, in which case, I think some of the benefits of balanced connection to the transformer are worthwhile. 

You asked "what is preferred".  If you own a balanced phono stage it would make no sense not to take advantage of that fact by using a balanced connection.  If your phono is SE, I would on the other hand not say that you should necessarily switch to a balanced phono stage.  Both types can sound excellent.