Personally I prefer the northern route, but not IFR. I would only go VFR in a single. I would use Sectional Charts to plan the routing and plan to stay over highways, specifically I-80. It roughly mirrors the old Oregon Trail that wagons used to take to cross the country on the way to the west coast. For the Rockies part it takes you north of the worst part and keeps you over relatively flat terrain ( high, but flat ). Plan a re-fueling stop somewhere in western Nebraska ( Scottsbluff, perhaps ) and then make it non-stop to Logan, Utah or another field on the western side of the Wasatch range. That way you won't have to deal with a very high density altitude takeoff from Wyoming. It will still be high, but not terribly high.
You haven't really said what part of the west coast though so it is difficult to make a suggestion from there. If you are going to the Bay Area just follow I-80 across Utah and Nevada. The last tricky part is from Reno over the Sierra Nevada to Sacramento, but again just follow I-80 and you'll be okay. If you are heading for Southern California then you can follow I-15 down towards Vegas and across the Mojave desert. The Cajon Pass and Banning Pass going into the L.A. Basin can be tricky in a light single. Go through with enough altitide that you can withstand a 1000 FPM downdraft for awhile. I can't tell you how many wrecks I've seen oon both sides of those passes. Preferably it is VFR and the winds are light then they aren't a problem.
If you are going further north of that, say Oregon or Washington then the routing would take you from Logan towards Boise, from Boise almost due west for awhile then north towards the Tri-Cities area and through the Columbia river valley to the coast. I haven't down that one in a single so it may be a little off, again just check a Sectional and you'll see a good routing that may be a little different.
Hope that helps,
Typhoonpilot