Of the two, I definitely prefer Xero - I find Sage to be a bit clunky and old-fashioned by comparison.
As your client has found, Xero does offer a free trial and they have plenty of online help guides, so my recommendation is to sign up for a free trial yourself and give it a go.
As for converting the data I know it is possible, I think there are some companies out there that will do it (for a fee). When I transferred my companies over, I took the opportunity to have a clean up of the records and started afresh with just the opening balances and kept sage around for historical reference if needed.
I think the main issue with Xero is the lack of phone support - they do have email support which, while pretty good, is quite slow - it can take up to a day to get a reply!