the Creedmor is a improved case design to the .243 in every way. Its a superior design period. I love the 243; I really really do. Grew up shooing them from yotes to hogs and deer. it is and super popular and for good reason. But the 6 creedmor is the better case and it just flat out works GREAT!!!!
Its slightly shorter case OAL and longer neck allows for both long high bc and shorter neck bullets to be loaded at ideal seating. Even the longest at the SA mag typical OAL. The sharper shoulder decreases case growth and trimming but still is easy feeding. New brass can be had with the small primer thicker head from Lapua and Peterson which will handle more pressure and loading's if you take care of the necks. (Basically forming the Creedmor from the Palma 308)
While you will certainly find 243 ammo in more local gun shops than creedmor, the big box outdoor store will have plenty of both. If you reload or plan ahead its a non issue.
As your in the Reno area you're at a bit of elevation; 4k-5K?? The 8 twist should work well. If you were closer to sea level or plan to shoot much close to it especially hunting winter such as in the NE I would have strongly recommended a 1:7-7.5. Especially for the 115 hybrids. If they are the VLDs then the 8 is perfect regardless. I shoot the 105 and 115 hybrids and am @sea level to 1-2K elev much of the time so went with a 7.5 and a 7 twist in another.
Be sure to get some pics up when its done and report how it shoots. That Krieger should produce some bugholes with those bullets. Its great your daughter has started to find a passion for shooting/hunting. I have had some great times with my girls hunting and plinking etc.. That should turn out to be , a great rifle for her. Light recoil, accurate, not heavy, good for all medium game.