Best thing I could tell you would be to get off paper at 100 yards and move out to longer ranges doing practical field practice. That is when you go to an area where you can spot shots of opportunity, such as a very small, well defined rock, range it, dial up or hold over your shooting solution, set up for the shot and shoot as you would in the field at a big game animal.
Record your shot placement, hit or miss and if miss, by how far you missed and what direction.
Move to another range target, repeat and again, record your impact.
Do this at varying distances and then take your impact data and compare it to the drop chart to see if you can tweak the drop chart to match up better with your rifles trajectory.
Tweak your drop chart and head back out to the range and retest. Continue until your drop chart matches up perfectly with your rifles actual trajectory.
Again, more of my opinion, groups are fine but they mean very little to the long range big game hunter. They tell us if we have a good consistant load but 100 yard groups tell us very little about a rifles long range performance. I have seen many rifles that shot well at close range that did not perform worth a darn at long range. I have NEVER seen a rifle that shot well at long range NOT perform well at close range!!!
As far as what is more important to long range shooting, velocity or accuracy, simple answer is both. Velocity however is useless without accuracy so you want as much velocity as is safe and your rifle will accurately handle but if it degrades accuracy, a bit more velocity is not worth it.
If you have one load that shoots 1 moa at X fps and another load that shoots 1/2 moa but is 75 fps slower, take the 1/2 moa load, 75 fps means nothing at long range if your consistancy is degraded.
Again, test at long range. I have also seen many rifles that grouped 3/4 to 1 moa groups at 100 yards that shot into the 1/2 to 3/4 moa group size at longer ranges. This is not typical with the lower intenstity chamberings such as the 308 but with any long range bullet, generally they tighten up in moa group size as shooting range increases.
Back to my main point, find a good load and get off paper and onto practical field practice shooting instead of punching paper groups. Your rifles barrel will last much longer and you will have much more fun with your rifle and learn much more about long range shooting as well.