Any others that I may be missing?
Only the one with the widest range of all....Heroics/Ros.
Before really starting to think about the figures make a decision on the basing. Baccus and Irregular come on preset moulded base sizes. If they do not match the basing you want clipping and re-arranging is a PITA.
Adler and H&R are individual so are much easier to arrange if you want to do anything unusual with base sizes/dimensions.
Once you have an idea on that then start looking at the vastly different styles and prices of the ranges. Also which uniforms are available as standard and which will mean conversion or substitution.
My preference for 6mm is Heroics/Ros. I prefer the slight proportions without the over exaggerated heads, hands and weapons that Baccus and even more so Adler suffer.