The Sierra 168, for all its virtues (very easy to get it to shoot well), is not at all a high performance bullet, ballistically-speaking (wind drift). It is outperformed by the Sierra 155 (and by all other 155 target bullets too).
An interesting tidbit about the Sierra 155 is that its nose profile is the SAME as the Sierra 168 (with one minor exception - the 155's nose extends just a bit further, and the hole at the tip of the bullet ("meplat") is therefore a bit smaller than in the Sierra 168. Where you can use this to your advantage is that if you know what works well in your rifle with Sierra 168s (e.g. seated to 2.800" magazine length works well, or .020" off the lands, etc), then if you try exactly the same setup with the Sierra 155, it will likely work well for you.
The back end of the Sierra 155 is the same as the Sierra 190. Being a shallower boattail angle than the Sierra 168, it will be lower drag. It also might end up being a bit fussier to get to shoot well, especially if your muzzle's crown has any flaws. Short-boattailed bullets (like the 168) are less sensitive to this, and flat-based bullets even less sensitive still.
So if a Sierra 155 will shoot accurately in your M14, you have a load that will ballistically outperform a Sierra 168 at 100 yards, at 300 yards, at 600 yards, and beyond. And it will have a tiny bit less recoil. And if you are a DCRA member, you can periodically get the opportunity to purchase Sierra 155s in bulk (1000 at a time) for very attractive prices.
But, a note to the Original Poster: if you're not shooting any further than 500m (or 600m), don't worry too much about the ballistic advantage of one bullet over another. Accuracy and availability (and price) are much, much more important.