The dividing line is something between having an armed Virgin Galactic - up and down, with some manoeuvre capability, or the X37 with orbital capability.
The choice between them depends on the role envisaged for the vehicle and the likely weapons systems.
A VG type, ground launch, alternative engines could be like a point defence aircraft, but at speeds and altitudes which offer enormous gains in weapon range; providing the detector systems can match that - not excluding ground based or other airborne detections. It would be expected to have an intercept capability against conventional aircraft at ranges in excess of a stand-off weapon launch and probably an anti satellite or even anti ICBM capability. There might even be an attack stand off capability (anti ship / submarine). This is typical for the requirements of large territory defence systems, N America, Canada, Russia.
The X 37 type would be more like a space battle platform, but with limitations of orbital mechanics. The ability to launch a reconnaissance vehicle on a selectable one orbit or even fully manoeuvrable orbital track could be very useful. This might be beyond a ‘fighter’ designation, except if the vehicle has a runway take off, followed by a choice to orbit or return to base. Again refer to the expected role.
Both options might require developed HOTOL type technology, and possibly alternatives to the SABRE engine. Yet the jet engine started life in two guises, axial and centrifugal, so why not a jet, ramjet, rocket, with a bit of linear acceleration for take off (with ski jump for upwardness).