The two young shooting guards are very easy to compare. They entered the league together by way of the 2008 NBA Draft. They're similarly built (Mayo's 6'4", 210 lbs, Gordon's 6'3", 215 lbs). And their numbers are comparable as well.
Before being relegated to a bench role in Memphis in order for defensive-minded Tony Allen to start, the former USC Trojan was actually more effective than Gordon offensively.
Through their first two seasons in the league, when both were starters, Mayo averaged more points, rebounds, assists and steals than Gordon. He also shot a higher percentage from three-point range and the free throw line and was less than one percent behind Gordon from the field.
While Gordon was given a chance to continue to grow in their third season in the league, Mayo's development was stunted by the lineup change that sent him to the Memphis Grizzlies' second unit.
Even still, Mayo's career numbers over his first four years in the NBA are not far behind Gordon's:
And based on the proverbial "eye test", it's hard to claim Gordon is a more naturally talented basketball player than Mayo.
So, from an economic standpoint, Mayo makes much more sense for Phoenix. Gordon recently signed the Suns' offer sheet for a four-year contract worth $58 million. They can probably land Mayo for half that price.
That will give Phoenix enough money to add one or two other solid free agents playing whatever positions at which they feel they need more depth.
To me, a duo of Mayo and recent Suns signee Michael Beasley is young, exciting, explosive and has lots of potential.