I've gotten to be a big fan of synthetic insulated jackets. They can be very warm and weigh very little. And they take a fair amount of abuse, they're easy to wash, are almost as compressible as down, and they aren't as sensitive to getting wet. It also seems like one jacket can cover a wider range of temperature conditions making it a little more versatile (or maybe I'm just imagining that part). I don't really know if the longevity of the insulation will be the same, though. I think down sleeping bags, for example, can last many years longer than synthetic ones.
I don't know those Arc'teryx models, but from their website the Atom SV you mentioned looks like it might be similar to my favorite, the Patagonia DAS that I mentioned above - a seriously warm hooded jacket. The other one, Atom LT, looks similar to a midweight MontBell jacket I use a lot which is also similar to a Patagonia Micro Puff (not to be confused with the lighter Nano Puff).
Arc'teryx can have a reputation for being high priced, but my limited experience with them (two pairs of pants), and what I've heard, suggests that generally their stuff is well designed and well made. So if I was in the market for a new jacket I'd probably look at those and try them on.
I wore my DAS parka to 19,545' in Yukon, Canada last June. OK, I know what you're thinking, "Summertime", but with the wind chill it was said to be around 35 below (C or F, take your pick). Admittedly, there were more layers underneath, but without that jacket I'm sure it would have seemed a bit chilly. For the record, down was used by others on that trip and they were happy too.
Let us know what you decide on.
