This has been my conclusion too. Switched to Singapore as a "core" for the up to date math terms, use Strayor-Upton and sometimes Ray's for extra or alternative methods. Strayor-Upton is even set up similarly (more cycles in S-U, but still the addition, subtraction, multiplication, etc. progression), so I find it fairly easy to sync, plus, anything not covered in Singapore but in S-U we will do over the summer. FYI I moved to Singapore this year after using S-U and Ray's for awhile.