Okay.  I've posted elsewhere about a member of our club needing to sell a Walther PPK at the swap meet we held this last weekend.  The primary impetus for the swap meet was one of our members has terminal cancer, and he donated all his guns and reloading equipment to the club, with the proceeds to go toward purchase of an electronic access gate.

This particular member is a retired engineer, and a pretty fair gunsmith.

Among other things, I bought a 30-06 he had just finished built on a K98 Mauser action (WWII bringback by his uncle).  It hadn't even been test fired yet (though that was remedied Sunday).  He didn't like the look of the fired cases, so he gave me back my check and took the rifle home to take the barrel off and check all the dimensions.  Possibly polish up the chamber.  When I talked to him this evening, he said he recalled that I might have mentioned that I already had a .308 (I do).  Since I do, and the 30-06 would be somewhat redundant, he asked if I might be interested in a 6mm-284 varmint rifle in place of the 30-06.  Same $$.

As I mentioned, I don't need the 30-06, but I have probably 400 rounds of brass for it, so that was a consideration for wanting one.  (I have at least a thousand 223/5.56 cases as well, without a gun - yet, but that's a different topic.)

Anyway, I'll call him Friday to discuss his findings on the 30-06.  If it cleans up okay, should I go for the 6mm-284 anyway, or stick with the 30-06 (which had the most awesome trigger I've ever pulled)?

6MM-.284 is interesting, but really after looking close at it you'll see it barely does much more than 6mm Remington, which is a factory load and cases are cheap.
Cases for 6mm-.284 aren't cheap, even when formed from fresh .284 cases.
Obviously you will have to reload for the 6mm-.284.

If reloading for a wildcat isn't a problem (does the 6mm-284 include dies?) and you've already got a 308 you're happy with, I'd go with whichever rifle is more personally interesting (which is completely subjective).

  Are you set up for reloading (it looks like you are)?  Go for it.  That round is a coyote widow maker.

I have to admit, I'm leaning toward the 30-06.  At least slightly.  That way my dad's '06 can go to my older brother. 

It really depends on what he finds when he pulls the barrel.

I didn't want to rain on your parade, just be realistic, but if you reload that 6mm-284 can be pretty interesting. I like 6mms quite a bit (<-understatement),
my first deer, prairie dog, and woodchuck were all shot with the .243. (In a couple of different rifles.)
Anything .243 can do 6mm-.284 can do in spades. It and it's close ballistic mates the .240 Weatherby and the 6mm-06 wildcat are very similar. All are really more of large varmint/deer calibers than pure varmint cartridges, and really shine with heavier bullet weights versus the .243.
But they will launch the mid-weight varmint bullets far and fast enough to give even the hottest .22 centerfires a run for their money.
http://www.chuckhawks.com/6mm-284.htm
http://www.reloadbench.com/cartridges/w6mm284.html
http://www.chuckhawks.com/240weatherby.htm

The 6-284 can be finicky to load.  It also burns up throats pretty fast. 
When a .243 throat goes you can rechamber, but to what do you rechamber a 6-284?

You could always just pull off the barrel, cut the shoulder back a thread, and recut the 6mm-284 chamber.

I haven't seen it, other than just in passing a couple of weeks ago when it was being assembled, but he said it was a fluted barrel (which I actually do recall).

And just to throw another potential monkey wrench into the pot, when we talked a couple days ago he said something about how he may have modified/wanted to modify it somewhat to reduce pressure and extend the life of the barrel.  Sounds like Ackley Improved, maybe?

any possibility of having it made into a different 30=06 family of cartridges, and then resize/reneck the 30-06 brass you have?  Have him build it as a 25-06, 35 whelen, etc.  Those won't overlap on your 308 nearly as much.

Or, if you are willing to wildcat, what about a 243/06  or 6.5-06
http://www.udarrell.com/243-06-ballistics-sierra.htm

http://www.accuratepowder.com/data/PerCaliber2Guide/Rifle/Standarddata(Rifle)/264Cal(6.5mm)/6.5%2006%20Page%20232.pdf

there is the 338-06 Asquare, but it is rare enough it might as well be a wildcat.  That to me sounds like a real great cartridge, and would really shine in the moments where a regular 30-06 would be a better choice than a 308.  (as in, 'leave that 308 behind, we are going after moose/grizzly, and the 308 just can't handle heavy 30 caliber bullets, so take your 30-06 and cook up some 200 or 220 grain hot handloads...well a 338-06 using a 250 grian or 275 grain bullet would be even better!)

there are a ton of really really good 338 bullets out there, not so much variety with the 358 for the 35 whelen.

It seems to me the 338-06 mixes the best attributes of the 30-06 heavy weights and the 338 winmag.