The Sportsworks rack uses a spring-loaded arm to hold the front tire. It is very easy to use, but flimsily made and it tends to slip off if not perfectly positioned. The arm on the Portland rack is hinged near the front wheel and swings up to grip the downtube of the bike with a quick-release jaw. The Sportsworks arm was the least secure, and we almost lost a bike when the arm slipped off. The security of attachment is not the only concern, however.
Ease of use provides almost the opposite ordering.
The arms of the Portland racks were difficult to get into the correct position, especially on bikes with front fenders, which provided little clearance between the front wheel and the downtube. There were also subtle adjustments available to the shape of the jaw which we did not notice at first, and which most bus riders are not going to have the time to examine and decipher. Considering how many bicyclist do not know how to operate their quick release tires properly, we regarded it as unlikely that everybody would figure out how to adjust and close the more complex Portland jaws correctly. The arms of the Sportsworks racks were probably the easiest to use, but none of us were willing to trust our bikes to them. Ease of use consists of more than just figuring out the arm mechanism.