Im in the same boat atm and concluded the Atrix is the phone for me (Don't forget to check the samsung galaxy W) simply because
ARC
pros: good screen quality, good camera
cons: 1GHz single core (1.4GHz single core on 's' isn't impressing me either), Don't like the width of screen, it's sony (last sony phone fell to bits)
Atrix
pros: Tegra 2 (nvidia gaming zone looks hopeful), dual core, webtop will be useful, finger print scanner is a nice novelty, qHD (is arc qHD?)
cons: Blur, locked to orange(current network but a negative for some)/T-mobile
galaxy W
pros: cheaper and cut down version of S-ii but still a good performer
cons: can't get a good enough deal on it compared to the atrix
HTC Sensation
pros: HTC are a solid manufacturer, very good phone
cons: I hate Sense. It bores me and is becoming bloated.
Blackberry torch/bold 9900 (currently using bold 9700)
pros: I like keyboard phones, BBM is pretty good (if I had friends with it)
cons: Missing HDMI out, lack of decent apps, no front facing camera, no guarantee on QNX coming to current devices, overpriced, Doesn't play nice with google calenders for me, Problems connecting to app world and receiving emails at times.
Nokia Lumia:
pros: free sat nav, epic looking phone, music service sounds great, fondness of Nokia
cons: No HDMI out, no front facing camera (from memory)
other: Windows phone 7 (doesnt annoy me, but may annoy others)
Galaxy S-II
pros: you actually have to ask?
cons: its too expensive for me =( ..........(tho i might go for it anyway)