I think the Keeley is the most common. I have one. It's pretty good. I've never heard the Allums one. I'd like to check it out. I'm having my other Rat modded to M'ODish spec by a fellow TGP member but customized to my spec. I haven't gotten it back yet.
I've thought about sending my Rat 2 to Keeley but I like it so much stock, I'm afraid of losing the original sound. I have played with a newer version Keeley Rat 2 and it sounded really good though, but he no longer mods the new ones. Does he do Vintage reissue Rats also? That might be an option for me. Haven't heard the Monte Allums in person.