There is a chance that both the Diamonback 21" and the trek 22.5 will fit the same since there are both XL frame size for each model. The Insight look like it has a more compact geometry so that makes the seat tube shorter. I would compare the geometry between the two but Trek doesn't have there geometry specs online. $300 is a big price difference and while fit is improtant there is no reason to spend a bunch extra when the difference MAY be small. there is usually a number of places like big sporting goods stores that stock the Insight even if its the basic one it will fit the same. I would at least try it out.