Wife's favorite piece of gear
Between the wife and myself, we each own one Mountain Hardwear bag and one Western Mountaineering bag.
In the argument of the Phantom 0 versus a WM Antelope, she and I agree that when splitting hairs and cost independent, the WM is the better product by a very slim margin. However, WM products are near-impossible to find on sale, even Backcountry.com's 15%-20% coupons exclude WM. On the flip side, Mountain Hardwear stuff is frequently on sale, and as a result this bag can often be purchased for $405-$415. For 90% of the population, we'd offer that the Phantom is certainly the better value and gives away almost nothing to the WM.
Of the four bags we own, the Phantom 0 fluffs up and has the best loft of all of them. It's absolutely my wife's favorite piece of gear. She is a notoriously cold sleeper, but has never been anything other than toasty and happy in this bag. She definitely appreciates the extra insulation in these female-specific Phantom bags.
If you're shopping at full retail prices, then the price difference between the two bags is marginal, and sure, get the WM. Otherwise, shop off-season and get the Phantom 0 for a steal.