Xor or single ended?

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.

Thanks to all that responded!
 My preamp is fully balanced but the tone arms were mistakenly made with s. E. Terminations!
my question was centered around whether I should have the terminations changed or just use adapters... I think the former seems to be the unanimous choice...:-)

I use balanced ( XLR) connection from my phono into the preamp, and then into the amp.  The result is abslutely dead quiet with fully realized performance.  I also have balanced headphones and separate balanced headphone amp which can be used easily with regular plugs, and there is an increase in performance using them balanced.