I ended up with an ST-3 as that was all that was available at the time my 6/1 was to be shipped. Much smaller, lighter. It's adequate for my needs, and at 5600 feet elevation it's a pretty good match for my 6/1, which often has a parasite load of my air compressor, decompressed. I have no motor starting loads bigger than 1/2 hp. I usually run at 1000 -1500 watts load. A load of 2000 watts is rare, and only in the winter when I'm running my shop lighting/heat lamps.
The upside to the ST-3 is that with an AVR it has less flicker, as the rotor has less inductance than the ST-5, and so it is more responsive in correcting for the speed change of the compression/ignition stroke.
For near sea level applications, I agree the ST-5 is probably the best way to go, for bigger motor start capability, and especially if you expect to be operating well over 2000 watts continuously in the summer. And especially if the freight is the same. George B was kind enough to break down an ST-3 for me into two containers so it could be shipped UPS. Saved me a bundle on my backup ST-3.