How large do you think their database is? Express versions of SQL can stuff only 1 gig of data into memory. So when databases get larger than that, since it can stuff only 1 gig in RAM, the rest will be paged on the hard drive...which is slow. The database size is 10 gigs (for SQL Express 2012). 4 gigs for SE 2008.
So it usually comes down to a performance limitation.....if they're smaller, they can likely start out with SQL Express. Once they start having issues with performance, move them to full blown SQL.
May want to take a look at their current database...and talk with Sage support about it, they can look at it and give honest opinions. Run some database maint utilities on it too, shrink it down, can probably prevent needing to go to full SQL for quite a while.