AXO feels a bit more restrained, like they didn't drink too many Monster drinks. Their selection is varied (mx/road/bmx etc), but smaller.
Klim is very plain, it seems geared less to motocross and more to the endurance/off-road side of dirt bikes
One Industries make fairly down the line jerseys, they have printed ones that don't go too crazy as well as some simpler designs in nice geometric patterns.
Fly Racing has a lot of prominent branding and huge colourful geometric patterns. They cover a bunch of fields; road, bmx, dirt etcetera, but I didn't bother looking through everything.
Thor have some of the most spastic, thousand layer/colour jerseys, but they seem fairly well though out. I mean that they look okay despite the fine line they run. They have some plain stuff as well.
JT Racing have jerseys that look quite simple in a retro fashion as well as newer ones. They don't produce retro jerseys per se.
Troy Lee Designs make both retro and new jerseys for motocross and bmx. their designs are bright and graphic, but overall quite nice.
Team Moto is an Australian based website. I have no idea why I saved this link, I think it has a good range of stuff probably.
BTO Sports is an American website that has a huge range of jerseys. A pretty good visual reference source.
The history of Go The Rat - http://www.gobadirt.com/stories/sx-mx/109-go-the-rat.html