Kendra, if your current WM bag is for 25F, then it is either an Alder or Sycamore, both of which are semi-rectangular bags. (Unless you have a model they haven't made for some time.) You wouldn't/couldn't buy a synthetic to put over a semi-rectangular; if anything, you'd buy a bag to put inside it. However, semi-rectangular bags aren't particularly heat-efficient.
Remember, sleeping bags don't produce heat. They merely trap the heat you produce. A bigger bag requires you to heat much more volume, then to waste more energy through the night keeping that extra volume warm.
Since you have a WM bag, my first recommendation for you would be a WM Antelope. Great loft, rated to 5*F, good for most people lower than that. 2.5 pounds, but ~$450.
Trying to keep it on the cheap-ish, check out the TNF Snowshoe, a 0*F Climashield bag. Of the synthetic materials available, Climashield has about the best longevity. Also good weight and compactibility as related to synthetics. ~$200, 3.5#. Edit: Note that you'll have significantly fewer years of use with the synthetic bag as compared to a down bag. Initial investment on synthetic is cheaper, long-term investment down is cheaper. But sometimes a budget is a budget.
There's arguably less risk of getting a bag wet in winter than in summer. Less humidity! No rain! No worries about a down bag in winter. The biggest problem people run into is sweating too much into their bags. If you're doing cold stuff for multiple days, consider a VBL... again with WM, check out their Hotsac. It's pretty awesome. Will also easily add 10-15*F to the bag if needed. If you're going to do the quinzhee thing regularly, perhaps a bit more exposure to moisture... I'd sleep on a ground sheet or something. Might consider a bivy; water-resistant sleeping bag shells are popular with some people, but do add to weight, bulk and price.