Hi Phil, in some ways I found them similar. Both sound natural and liquid but with no compromise on speed, attack or dynamics. The strong suit of the ARC was the width of its soundstage (not that the Allnic is lacking in that area !) but I preferred the Allnic as the sound seemed a lot more holographic and instruments had greater density, fullness and presence... it's a very sumptous seductive sound that you very quickly get lost in and it seems to envelope you from all sides. I preferred the perceived build quality as well. It's very very quiet in my system. The ARC is good too, perhaps more neutral if that's your thing but for me not as enjoyable and not the sort of sound I aim for. I have also heard exceptional things from friends/associates about the Thrax Orpheus, Ypsilon and the TE Mastergroove but the former two are too expensive for me and the latter is solid state.
As for tonearms I use the Reed 3p (used to have the Raven 10.5) and carts XV1-s, Shilabe and Puritas.