Easy to group (based on my observations):
by cost - VS/JPS low, Beck med, IM high
by chassis - VS/JPS pan (AC mainly), Beck tube (AC to Subbie), IM custom (AC to Subi to Porsche 911)
by Style - VS std/wide/ultra (is wide even still avail?), JPS Speester/coupe, Beck Speedster starting coupe (spyder and GTS 904), IM speedster/D multiple engine/suspension options up to 911.
by performance - VS pan/swing axle, Beck tube, IM Porsche under pinnings
by Finish - VS/JPS nice acceptable quality (tight quarters interior), Beck great quality (roomy interior), IM high end production quality (roomy fitted interior).
By quality? by support? by innovation? by weather sealing? by resale value?
Seems like it all boils down to "you get what you pay for."