A client with two websites

Hi guys,

I have a client who has two websites. One is on wix and the other is on wordpress. I have no idea how long either of these have been around, but they've been around for some time. They're both indexed and the wordpress site is the main site, however, the wix site still holds SEO value and doesn't seem to be penalizing them from what I can tell. The wix site holds just as much information page-wise as the wordpress site and the main site has a lot of backlinks pointing from the wix site. So, my main concern about taking the wix site down is loss of backlinks and potential SEO value. My initial response was to move over any copy we could that would prove beneficial and then just do 301 redirects. What're your suggestions?