it really seems like you have your heart set already. you seem to be asking a lot of questions pertaining the 6.5 grendel. sooo. do yourself a favor, and do some homework and BUY WHAT YOU WANT. it you buy something that everyone else says you need, you will never be TRULY happy with your purchase. make sence??
the 6.5 grendel is a very good round. it will out-perform the 308 out to 600 yards. and that being the distance you are shooting, i say it will be vary ood to you. i do not personally have one, nor have i shoot one. this is all based on months of research, and asking questions. i want one, but dont have to dough right now. i also have a 7 rem mag in the works, so i think that they long range nich is filled. for now at least.
i would say that the 24" grendel barrel will be the best at squeszzing all the potential out of the 6.5 round.
do you already have an AR-15 platefrom , or lower? if so, you can get the upper, grendel's very own DESIGNATED MARKSMEN upper, which is a 24 " ss barrel with all the bells and wistels. though this does run 2300.00$. thats alot, but if you are wanting to win in compititions, upi have to spend some money, tiem and lots of pratice pratice practice.
hope this helps, brian.
also, check out
http://65grendel.com/ and go to the forums page. theres a lot of great guys there that will more then be happy to answer any question you have. i know, i have asked a lot there myself. brian