Probably a stupid question, but for what use do you intend this rifle?
I don't really want to spark a debate here, but - in my opinion - either of these cartridges are considerably overpowered for deer. Over the years, I've watched the .300-.338 magnums turn many good shooters into "flinchers." Combining opinion and observation, unless you're intending a western (elk) hunt or a moose hunt, I'd look instead for something from .270 to .30-06. If you MUST have a "magnum", the 7mm RM's already been mentioned here.
Regarding short- versus long-actions, most hunters can't capitalize on any weight savings afforded by a short action because it's really minimal, and any weight savings is quite often more than made up by too many Big Macs around their midsections. Yes, you might gain a bit more barrel length out of a short-action, thus negating any weight savings, for a very modest gain in velocity. Want a light action with a long barrel? Find yourself a Ruger No. 1.
I don't/won't own one, but everything I'm hearing at the range from those who do and who speak of it objectively is that unless you're reloading (and all of them do), there's not an appreciable advantage in the .300 WSM over the old .300 WM. The remainder of my range contacts are with guys who can cite almost every marketing point from any ad that's ever been published about their spiffy new .300 WSM.