I have both the Trek interchange system and the Topeak sliding rack system. I like them both, but I'm not sure what you mean by the Trek not having the sliding rack? The interchange system accessories snap on actually a little easier than the Topeak. You just push them in and snap the back. The Topeak you have to slide and then snap. The Trek rack also holds my basil basket that has a quick release as it is a little sturdier than the Topeak, and also doesn't have the track in the way. I like both systems. I got the Topeak rack because I wanted a smaller bag for my commuter bike, and the small bag I got only fits on Topeak racks. The Topeak bags are NOT interchangeable with other systems. They have no velcro attachments so they only go on the Topeak rack. The Trek interchange bag on the other hand can be put on racks other than the Trek one.

They're both very secure. I like the compactness and looks (HA...my bottom line!) of the Topeak, and the ease of the Trek.

Hope I didn't muddle your decision any more. Maybe you should go to a shop and compare?