In reply to monsoon:
Fit is the main difference, if you ignore the price.
Patagonia tends to have a roomier fit and will certainly be a better choice if you're a slightly stockier gent, whereas Arc'teryx tend to have a more athletic (read smaller) fit.
In terms of quality you'll find them to be similar in my experience.
My advice would be that unless you're operating at a very high level, avoid going for the 'Fast and Light' offering from either brand and instead look for the 'Bombproof in mixed weather' jacket.
From Patagonia this would be the Triolet, from Arc'teryx the Beta AR.
These jackets will last much longer, run 'quieter' (i.e. less rustling) and perform just as well for a weight compensation of 100g or so. They're also cheaper so it's really a win win.
There's also other aspects to consider such as company specific policy - by way of example Patagonia has an impressive environmental stance and are very happy to repair their products, meaning that a broken zip or small tear doesn't put paid to £400 of jacket. I'm not aware that Arc'teryx carries out a similar service but I'd be happy to be proven wrong.
Personally I'd go for the Triolet - my girlfriend has one and it's bloody excellent. She comes down off the mountain after a day of damp, sweaty off piste in crap conditions and her down jacket is bone dry underneath the shell.