I need a new stove, and although it isn't winter, I want one that works up here (Quebec) when I camp in the winter (commonly -15C).
My last stove was white gas and worked like a beauty. Can I safely assume that all white gas stove will do under these conditions? Or is there some reason I need the bulkier XGK-EX?

The reason why the engineers at Cascade Designs (parent company of MSR) chose not to make any of their liquid stoves burn both canister and liquid fuels is based on performance. I can tell you it's been attempted, studied, tested, engineered, thought about, discussed and argued more than any other topic around CDI's production facilities. What they have found is that you either have a stove that performs near flawlessly in extreme weather (like you would find in the Himalayas at their worst) or one that is 'ok' at doing everything. They have come to the conclusion that canister stoves need to burn canister fuel only and liquid fuel stoves need to burn liquid fuel only. The reasons are many. Liquid fuel stoves, in the opinion of the MSR engineer, should be as reliable as possible. The more 'features' you add to the liquid fuel stove, the more opportunity for failure when it's needed most. Often liquid fuels are dirty, contaminated, not suited for using in a lightweight backpacking stove, and, in some cases, barely burn. MSR believes their liquid fuel stoves must work with these types of fuels. Even if you're talking about white gas only, there are a million different ways white gas can be contaminated. The overall weight of the stove must also be taken into consideration and the stoves that burn every fuel, including canister fuel, are generally much heavier. Canister fuels are generally contamination free. However, the fuel is much different than one would expect from a liquid fuel. What is not known when manufacturing stoves is that valving, the size of the jet hole, the distance between the burner head and the bottom of the pot, the length of the fuel line, the flame adjuster, etc. are all different on canister stoves vs. liquid fuel stoves if you want optimum performance. What the MSR engineers have not been able to find is the obtaining of optimum field performance from one stove that burns every type of fuel available. I've had the opportunity to try out most stoves available today in the field and my conclusion is the same as those of the MSR engineers...either you get excellent performance from stoves that are specific to a fuel type, or you get mediocre results from a stove that attempts to be everything for everybody (multi-liquid fuel, canister, lightweight, packable, excellent pot supports, easy to use, stable, wind resistant, etc, etc,). This is currently the status at MSR. However, things might change as new technology and discoveries are made.

There are a couple of things to keep in mind when considering both stoves: 1. Simmerlite - this stove was designed for lightweight, cold weather backpacking. If you have access to white gas (which is the case in Quebec) then this stove will be adequate for any winter conditions, no matter the temp. It isn't as bomber as the XGK-EX or has hot burning, but is easy to prime, self-cleaning, light weight (8.5 ounces (not including fuel bottle)), and is more capable of simmering than the XGK-EX (not as good as the DragonFly however). The Simmerlite is also less expensive and very quiet while in use. 2. XGK-EX - This stove is the best performing winter stove on the planet. It was intended for high-altitude mountaineering, but with the new updates, is actually fairly light and easy to use (only 2 ounces lighter than the Whisperlite). It's best features include it's pot supports that will support a 10L pot if needed, a very stable base, burns incredibly hot (for boiling snow), is an easy stove to field maintain (important if your life depends on it), the best multi-fuel stove on the market (can burn almost anything liquid and flammable in a pinch), will be the last stove you buy (super durable), and is fairly light (14 oz. not including fuel bottle). It's not-so-good features include it being super noisy, hard to simmer, heavier than the Simmerlite (but lighter than the DragonFly), can be too hot if not melting snow or boiling water, it's a fuel hog, and may be overkill for an overnight trip (although I know many who use this as their only stove). The old XGK was one I seldom recommended unless your plans included high elevations and melting snow. However, the new XGK-EX is a whole different animal. I would recommend it to anyone who wants a stove that they can absolutely rely on, will last them several decades, and doesn't mind the noise. It is truly an awesome stove! It's important to note that the XGK-EX is not compatible with canister fuels. Multi-fuel means that it can burn diesel, gasoline, kerosine, white gas, or other liquid fuels (not alcohol fuels however). Canisters are not compatible with the XGK-EX!

This would do just fine for you needs! I couldn't recommend the bulkier XGK-EX unless you were planning on a longer time period, high elevations and a great number of people.