Bottom line is if your business is going to be bookkeeping for clients who themselves will be using the software to import bank statements, produce invoices or try and submit VAT Returns then the client will be happier with Xero than anything else, so training in Xero makes sense.
However, if...
22 years, 100s of clients myself, many many of whom have been glad to move off Sage, nobody's ever asked to go on it from Xero Not user friendly for the business owner, pricey, clunky, a dinosaur bit of software. Even on the Sage Facebook page Sage people themselves say Xero is better for...
Not in the small business market - Xero and QuickBooks are miles ahead. I've just recruited a girl who left her last firm because she hates Sage and they way her employer foisted it upon their clients.
I'm an accountant and I echo the advice re AAT. Furthermore, it is essential for new bookkeeper to be proficient in either Xero or QuickBooks accounting software, they are the top two and Xero is best. With Making Tax Digital it's a must.