Are you asking what brass to get, or which chamber to spec?
As I understand it, there are reamers out there that are cut specifically for Norma or for Winchester brass. They have slight differences in case head width. (Lapua should be the same as Norma) If you have a tight chamber cut for the tighter of the two (I can't remember which is which now) the other might be difficult to use. If I'm off base on that someone please let me know, but I'm almost positive that is what I read, although I cannot find the original source right now.
You might want to factor in this when considering Win brass (lifted from 6mmbr.com):
"Winchester Brand .284 Win Brass STATUS REPORT
Rumors that Winchester has stopped production of .284 Win brass are false. We contacted Olin (parent of Winchester), and here is the official story: "Winchester .284 brass is a 'seasonal production item'. This means it is not produced continuously, but rather is made in batches during the course of the year in response to dealer orders. Winchester did not receive sufficient orders for the 2005 production run of this caliber, so neither loaded rounds nor shellcases will be produced this year. The next possibility for a production run would be June of 2006."
So, production has NOT been discontinued, but we may start seeing supply shortages by late 2005. As of July 15, 2005, there are many vendors who have Winchester .284 Win brass in stock, including LockStock.com ($37/100) and ImpactGuns.com ($17.12/50)."
Who knows if Winchester will be difficult to come by in the future, its hard to say.
My 6.5/284 should be done shortly. I had to make the same brass decision, and there's 100 pieces of Laupa waiting for it now. I've taken some measurements on it as well as on some new Winchester Bulk 30-06 brass as they came out of the box.
For what its worth -
Lapua 6.5/284 (100 pieces):
Length to shoulder ES = .003"
Length to shoulder SD = .0006339"
OAL ES = .001"
OAL SD = .000278"
Winchester Bulk 30-06 (50 pieces):
Length to shoulder ES = .005"
Length to shoulder SD = .0009707"
OAL ES = .0075"
OAL SD = .001466"
All the Lapua cases looked like this:
Some of the Winchester cases looked like this:
But others were more like this:
(If you can't easily tell, this neck isn't even remotely close to being not just round, but roundish. Under close inspection it appears to be a pentagon with some generous radius's.)
Now I know that both the measurements I took and the condition of both the finish and the neck concentricity can be taken care of with brass preparation. But it makes sense to me that if a company is producing high tolerances in the many areas I can easily measure, they have a good chance of producing it in the ones I can't (say, wall thickness).
Hope this helped,