I will assume he is meaning .308 (most common). If thats the case, it depends on purpose.
If shooting palma, no option, has to be 155gr
If routinely shooting beyond 600 meters, then I would recommend the 175gr, its better past 600 as it doesn't suffer the transonic problems that the 168 does. (keep in mind, if you have a 1:12 twist barrel, you might be border line in being able to stablize the 175's).
If you don't plan to go beyond 600 very often, then the 168's are easier to find, and proven. Also may be the only option if you have a 1:12 twist.