The main difference between the Rhyno and Singletrack is that the Rhyno has a brake track. It's a good box-section rim that is cheap because it's an older model. DT makes a nice rim, and the 5.1 is a bit beefier than the 4.1 but not a full-on FR rim, and 40-50g lighter than the Rhyno. Mavic EN 321[?] would work too. To be honest, once you get a decent hub, it becomes 6-of-one-half-dozen-of-the-other with regard to the rim. Going to 36H and using a good builder [very important] with any of these rims would get you rock-solid set of wheels.