If all you're ever using your Garmin for is uploading routes and tracking training data, then the 305 is a better fit for you. If you need to view a live map, or need to pick a route while you're out and about, then stick with your current computer. Also, your current Garmin works in the car, on hiking trails, you name it -- while the 305 is really only useful on your bike.

For training or fixed routes - go with the 305
For everything else - stick with your 60 CSx