Sidelocks are widely available in all/or most calibers up to .58+.
Most In-lines are .50 cal, although there are a few in .45 and .54 with Knight offering a .52 custom.
Target or hunting or both and for what?

Before you read any further, I would offer that I would rather have a higher quality rifle in a lesser popular caliber than a low quality rifle in the most popular caliber.

45 pro's
-More accurate for long range silhouette shooting.
-Easier to work up a small game load.
-Less recoil.
-Less popular, which means that great deals can be often found on quality rifles.
45 con's
-Less popular, but .45 is not going away.
-Considered the minimum for large game, although the 45 with a 350-450+ grain conical will do just about anything one would want in North America.
-Considered too small by some states for big game (Elk and so on).
-Less availability of rifles, projectile and accessory availability.

50 pro's
-Most popular.
-Widest range of projectiles and loading combinations.
-Widest variety of available rifles and accessories.
-Considered the minimum by some states for big game

50 con's
-None really as it's the most popular, other than the "It's got to be a .50 or it's not good" fluff

54 pro's
-Among the best in terms of ballistics.
-Considered by many to be the benchmark for big or dangerous game
-Less popular, which means that great deals can be often found on quality rifles.
54 con's
-Less popular, but .54 is not going away.
-Less availability of rifles, projectiles and accessory availability.
-Considered effective range load out to 125-150yards max.

My .0002