Since your getting a new caliber anyway go for a custom die set or at least the seater. 21st century has a good price on their arbor press style which is the way to go it’s custom to your chamber and bullet.
https://21stcenturyinnovation.com/buy-online/ols/products/calibrated-bullet-seater-die
Bryan Litz talks about runout effecting accuracy in one of his Applied Ballistics books(can’t remember which one) all of them are worth reading. Whidden dies are reasonable for what you get and they use Redding, Wilson or SAC bushings. Off the shelf Wilson has the finest, they don’t try to make everything like other company’s their focus is dies. Attached is a good article explaining how runout effects the bullet and some testing as well.
A simple test of sorting ammo and a shooting session at the range will show how bullet runout can affect the accuracy of your rifles.
www.rifleshootermag.com
For me personally when I switched over to a arbor press seater it tightened up the groups say a good 1/10th MOA but most importantly it significantly cut down on flyers. Ten shot groups at 200yrds taught me a lot. I was spoiled with a virtually wind free(mornings anyway at Usery Pass Mesa, AZ) range to test on. I feel it was giving the bullets twice as much time to screwup vs the standard 100yrds.
Pros for a custom die: Working your brass the least amount while still able to feed smoother than just neck sizing.
Less runout in several articles I’ve read on 6mmbr.com I’ve had to take my ammo to someone with a nice runout gauge to checkout but the arbor press/ inline seater definitely helps. If your chamber is custom in anyway and really if it has less runout than a factory chamber does then it “needs
” a custom die.
Cons: The die is sort of married to a specific chamber, fine for me using a Compass Lake chamber in a couple rifles that works well. More expensive but at least subtract the cost of the off the shelf die from the cost. If you go the custom route do yourself a favor and shoot a factory(virgin brass) round with most importantly the brass your going to use and at least the same projectile weight to simulate all the same conditions. I’m not sure it makes a big difference or not but my OCD wouldn’t have it any other way. What caliber are you going for? I’m a simpleton with my .308 Winchester and .223 stuff. Thinking about a 6mm ARC if I ever start seeing the ammo on a shelf.