Too far to justify the "safety" against winter weather
nishira-
I'm with the others in saying I would look askance at adding additional distance to "avoid" weather. As has been correctly noted herein again and again, I-40 is prone to ice, whereas the northern routes are subject to snow + the best snow removal efforts on the planet. Plus, you take the Southern route and that "earns" you a chance to see Southern CA and ride I-5 through CA. The traffic congestion alone out there would drive me to alternate routes. Traffic is more dangerous than winter weather, in my opinion.
I'm very familiar with I-90 in MT. There are at least 4 fairly high passes, and that's where the weather will be the worst, and the snow removal the best. Most passes will require chains during the snow removal periods, and I'm assuming your Penske is a dually, so conventional chains are not going to work. Still, I-90 would be next to last on my list (I-40 being last).
I'd do the I-80 route in a New York minute. Don't sweat the winds. Just stop for a while if it blows too bad. Shoot, the 600+ miles you'd save earns you a full day to wait out some winds or snow, anyway. Plus, that 600 miles will cost you $260 in fuel at 8 mpg and 3.50/gal. I'm very familiar with I-80 from eastern NE all the way to Salt Lake City and into NV. That's the way I'd go. You've got a fairly low pass in WY at the Continental Divide, some fairly high desert plains from there to Evanston, then canyons to descend to the I-84 split, and you can avoid Parley's Summit between Park City and SLC by staying in I-84 to Ogden. I am unfamiliar with the balance of the route to SEA, but would imagine you'd have only low passes until you reach the Cascades immediately east of SEA.
Foy
I hope leary doesn't mean scared
Seriously, I'm impressed that you are being careful to assess conditions, evaluate your needs, and desire to be prepared. All good things. However, I also think that you're stressing overly much about this. The roads very rarely get so bad that you'll be in real danger. Just use common-sense, adjust your speed for the conditions, have gear ready to use in case you need it (and know how to use it), and then go for it.
Of course, common-sense means that you will pull over and rest if you get overly tired, that you will get out and stretch your legs every few hours, and that you will stop driving for the night if you feel the driving conditions aren't comfortable for you. During this time of year, this might mean driving more limited hours so you're driving in daylight. And, if the weather appears to be turning bad, check your radio for weather reports and/or call that state's DOT for an update. It's better to stop early for the night and wait out a storm than it is to press on ahead sometimes.
Anyway, be prepared but don't be overly stressed about this, OK? People travel these roads all winter without incident. There's no reason to assume that you'll have any more trouble than they do.