I don't have experience of any of these payrolls, but doubt you will get much feedback since they all appear to be very new suppliers. Personally, I would worry that one (My Paye) doesn't even provide an address on their web-site - which is always a concern with on-line services.
Although not essential, you might also want to consider which are accredited by HMRC as meeting the current standard - which only appears to be The Payroll Site - that at least covers the 2012-13 year.
However, if you wish to try any, I would certainly ask them whether they support the new PAYE regulations (RTI) - and, if not, when they will do so (as this will be a fundamental requirement from next year).
Finally, I'm not an accountant (so have no relevant opinions on accounting software), but Xero appears to be a New Zealand company that has only been around for a few years - so I would ask about their commitment to UK legislation (e.g. for VAT, CT, etc).
As you can probably tell, I'm a mite suspicious about the wisdom of placing responsibility for such important aspects of your business with companies/software about whom little is known - and where it may be hard to establish communications and/or get resolutions to problems.
However, any one of these may precisely meet your organisation's requirements. The key, as always, is to identify what these are - so that you can have a sensible conversation with potential suppliers.