There are plenty of simple explanations of Steem (liquid, can be transfered and traded at anytime), Steem Power (locked up Steem in return for influence on the platform) and Steem Dollar (1 SBD can be converted anytime for at least a dollars worth of Steem, offering certainty regarding it's minimum value).
"Better" investment, is completely subjective however by the metrics that Weiss are using I can see why it ranked higher. It a question each of us to decide whether Weiss ratings hold any weight or not.