If you are shooting 100 thru 500yd, barrel length is not really critical for .308. But the .223 will still be viable at out to 500yd (and some), and in that case, 24" is a good length, and 26" would be better.
Piece of advice..., Over the past 3 decades, I have found my opportunities and motivations for shooting extreme distances have amounted to probably less than 10% of my overall shooting time.
It's a hoot, no question. But for practical purposes, I am reinvesting most of my own shooting investment in .223, and I no longer own any rifles chambered for .308; haven't since about 2005. For LR, I shoot .260 (far more capable at 1000yd than .308). When I ended my 1000yd experience, I held over my .260 preference, but it took some time before I stepped down to chamberings that are more practical for precision shooting at the actual distances where I am now doing it. Once I decided on those chamberings, it become vastly apparent that I had been wasting a lot of resources up until then. In terms of practicality and efficiency, the .308 is impractical at short distances, and only marginally efficient at 1000yd.
For 30cal, I shoot .30BR (don't go there unless you are both unreasonably obsessed, and rich), and anything less obsessive, I do it all with .223.
I am also developing (on the back burner) loads for my .280, in the extremely unlikely scenario where I may need to go beyond 1000yd. For hunting, my .280 shoots so flat that ranging errors become far less significant. If a .280 won't drop it, my primary mistake was in considering the shot in the first place.
Barrel twist is important too, for the .308, 1:10" should handle essentially all useful bullet lengths; and for the .223, 1:9" should handle all practical bullet lengths.
Finally, using commercial ammunition is overly expensive, and seldom allows the rifle to perform at it full potential. Handloading should be a priority that comes well before shooting beyond 100yd, or with an LR-capable chambering.
Greg