Gem-Lac is a Gemini Coating's product...I no longer use their products because they seem to think that one can use an air dry vinyl sealer under their pre-cat systems....sure...and a year later the finish starts failing
...I had a discussion with the VP about it and walked away less than satisfied over the whole experience so I refuse to use their stuff any longer...anyway, SW offers a traditional nitrocellulose lacquer and a pre-cat lacquer system....their air dry Hi-Bild system looks very comparable to Behlen's stringed instrument lacquer (at least on paper)...their pre-cat HI-Bild system has a lot higher solids % than the Behlen's does and I would think have a detrimental effect on the clarity and depth of the finish...on the flip side the pre-cat system builds dft (dry film thickness) quickly and is a pretty durable finish though I can't say how it would sound on an instrument...also, because it is a catalyzed system, the use of retarder doesn't affect the overall curing time nearly as badly as it can with an air dry system...
sorry, I can't answer your direct question about which is 'best'...
I can say though that you will most likely have problems finding a one gallon container of either system as most of their stores (at least in my area and from what I've heard certain other areas) only stock the 5 gallon units, which for a pre-cat system can be very wasteful as the shelf life is 6 months or less once catalyzed in the container....