although I hear the .30-378wby is their best seller, by amount of ammo sold, I can't imagine it being so popular, though guys are obsessed with the biggest fastest etc...

Personally I'd never buy a .30-378, I have .340wby, and would consider owning a .300wby, but not that monster.

So of the two, .300 RUM and .30-378 my pick would be the .300 RUM.

I thought the .300RUM was more of the match of the .300wby though, but perhaps handloaders can load the RUM even hotter than the .300wby.

Either way who needs to shoot a 180gr or 200gr bullet that fast?

Are you looking at these for a specific reason? ie long long range shooting/hunting? 500yds plus?

Not that it's apples to apples, but my .340wby with 200gr bullets will at least come out the muzzle quite a bit faster than a .300wby shooting 200gr, and even faster than the .30-378 w/200grs. Of course most 30's will be shooting 180grs, and most .338's will shoot 225 or 250grs.

So my advice/choice would be just about any other 30 than the .30-378.
I really see no need for anything bigger/faster than a .300wby and/or .300RUM

Please tell us the reason you want a superfast .300 though, perhaps you can get away with something smaller and still fast, like a .270wsm or .257wby?

Perhaps a 7mm Rem.Mag. loaded with some high BC bullets can do the job down range too, ie more energy at 500-1000yds than even a .300RUM?

I think I'd be comparing .300wby to the .300RUM though, and my choice might be the .300RUM of the two.

Personally though, I just think there's something cool about a .338 mag, perhaps its that they're less popular, ie everyone and their brother has a .300mag, and claims their version is the best. So a fast .338 like the RUM or .340wby is pretty sweet, I've heard that .338RUM's kick is particularly brutal, ie fast and hard, though my .340 is not worse than my .300win.mag, and perhaps even less so and that's without a muzzlebrake! I believe you can handload that .338RUM quite a bit faster than factory loads, but again that kick may stop you from loading it that hot.

So it's just my opinion of course, but I think the .340wby is better than both the .300wby and the .30-378wby. Course the .340wby and 7mm Wby are their most normal cartridges IMO. Ie the case that pushes the .257wby ie 100gr bullets, is also the same case that pushes 175gr 7mm bullets. ie their biggest caliber for case cartridges...

it's like the .375wby vs. the .378wby, which I think they've kinda killed off their 378 on their own doing nowdays, even though the .375wby is slower than the .378, I think most would rather have and say the .375wby is the better cartridge of the two.

And while you're at it, what about the .338 Lapua?