Depends on what you want to do with it. Normally if you wanted more capacity between the fence and bit, you could just move the precision plate farther back on the MFT, so the 17" would be just fine, without adding the extra weight and bulk of the 25. But if you found yourself performing operations where you needed to quickly move the fence 50mm from the bit, then 500mm and repositioning the precision plate was too time consuming, having the longer LS25 would speed up the process (and if absolute accuracy was a concern, that would obviously help as well).
In my usage, the 17" is fine. It's a rare event when I need to push the fence more than 10" back from the bit and when that happens I'll either reposition the precision plate on the MFT or use a rail mounted router (for mid-panel grooves and dados).
