I was trying to make the same decision between the 243 AI and 6mm AI. Well, I decided on the 6-284. I will get much more velocity with 6-284. Plus, you can use excellent brass and you don't have to due too much case prep except neck down a 6.5x284 case to 6-284. I will be using a Krieger 30" 1-12 twist, I will be using 75grn V-MAX and 87grn V-MAX bullets. This rifle will be mainly used for 500 to 800 yard varmint hunting. With this chambering, I will be able to get around 3850 t0 4000 fps with a 75grn V-MAX and around 3700 to 3850fps with 87grn V-MAX. I like using the poly tiped bullets when hunting varmint, the explosion is an awesome thing to see. Others like to use the larger 6mm bullets such as the 107 Matchking and 105 A-MAX's. Those bullets have better BC, but they don't compare to the Ballistic tip or V-MAX bullets when it comes to the "mist factor" The 87grn V-MAX has a BC of .400, pretty darn good for a poly tipped bullet, I will most likely end up using that bullet in this rifle. As I mentioned before, I should be able to push the 87grn V-MAX at around 3850 fps. This is going to be a devistating varmint rig. If I were you, I would go with 6-284 over all the other 6mm choices you have mentioned. The 6-284 is a better round than the 243 AI and 6MM AI, you well get much more velocity. The 6-284 and 6-06 are similar velocity wise, however, the 6-284 is easier to prep and you can use better brass with 6-284. Here are the specs on my current project;
1. Chambering, 6-284
2. Nesika Bay Model K, 1.450 DIA action
3. Krieger 30" 1.250 tappered to 1.00 at muzzle, 1-12, throated for 75grn to 90grn bullets
4. McMillan A-5, adj LOP and thumbwheel cheek piece
5. Vias Muzzle brake
6. Jewell 2oz trigger
7. Davidson base with 20 MOA added
8. Kelby rings
9. Nightforce 5.5x22x56mm NXS
10. Teflon coated all Matte Black
I am still waiting for the parts, they should be here soon. It's already been 4 months from the time I ordered everything. Once I receive the parts, I will pick the gunsmith for the project.
Good luck with your choice and let us know what you decide.