I already have a .338 Win Mag in a Ruger M77. I was just trying to convince myself that I didnt need it anymore.
If you have both then you don't *need* the 338 anymore.
I basically agree with EastHunter if your rifle is for hunting the big bears only. The 338's advantage is it will make a bigger hole. On penetration... it's often true that slower bullets penetrate farther, but the reason for that is because faster bullets will deform more and possibly loose morer mass, neither of which is particularly a bad thing. The laws of physics are very inflexible, and all things being equal, a faster bullet will penetrate farther than a slower bullet.
Four bullets come to mind when talking about big bears. Nosler E-tips, Partitons, Barnes TSX's and Swift A-Frames. I would pick the E-tip over the TSX and the A-Frame over the Partiton.
I might loose, but I would be willing to wager that a 180 E-Tip out of a 300 RUM will penetrate farther than 200 E-Tip out of a 338.... or a 200 gr A-Frame out of a 300 RUM will penetrate farther than a 250 gr A-Frame out of a 338. Which would kill a big bear better considering wound channel, etc? Probably the 338. Would I feel comfortable with either? Yup. The one advantage to the RUM would be a longer shot, say 300-500 yds if it came up. The 338 WM would be loosing a lot of velocity and momentum appraoching that distance. Always going to be tradeoffs.
Mark