The eye fatigue with a spotter is fairly common. It has to do with using just one eye (we are a binocular vision species). Some can counter that with a patch, some by putting some sort of a extension over the eyepiece to block the off eye, and some just are not bothered by it. I'm seriously considering a Big Eye binocular. I am very likely going to start by getting one of CameralandNY's $529.00 Minox 15x58 ED's from Doug. I have no extreme or hard usage for them otherwise, I'd be looking at the Vortex 15x56, or the Swaro 15x56.
The 15x58 Minox got some real good comparative reviews alongside Leica and Swaro high magnification glass, and that was before the current ED edition Doug got Minox to make for him. They will be a heck of a lot easier to transport than a big porro.