There is no clear cut answer, as always.
For example, Japanese cruisers are usually worth their weight in gold: very fast, good range, reasonable protection, excellent surface capability. On the other hand, the Commonwealth has a host of mostly useless cruisers. One more, one less, where is the difference ?
Now, there are other things to ponder: a destroyed naval unit takes at best one year and a half to come back on the map if you need to rebuild it. A damanged unit takes four months...
Suppose you have XD to assign, and you have a battleship and a cruiser: I usually would put the X on the battleship if its defense is 1 or less, and the D on the cruiser: this way, I have a 90% chance to keep both units. But if I feel that the cruiser is anyway worthless, I'll assign the result to it. An excellent example is the Yamato: with a defense of 0, it just cannot be sunk by one X alone. XD only has a 10% chance of destroying it. It takes XX to sink it with certainty...
The choice may also not be the same if you are at the beginning or at the end of the campaign: An Italian player, with few build points, may prefer to keep his battleships in good shape and attrition his fleet with losses, considering that he doesn't have the material means to repair two ships...
Well well well... Yes, Wif 1000 choices also do extends to such trivial things...
Now conventionnal wisdom also says that you should always avoid any naval combat where you don't have planes! And surface combat is usually bad, because it is bloody and unforgiving. Both sides will suffer. In naval air, your fleet may well end up unscathed (see my AAR for some naval combats examples)
First, having planes will force the opponent to spend precious surprise points to avoid naval air combat.
Second, you get to choose some of the opponent's casualties.
You take it when you want to attrition the enemy fleet and you have a greater firepower ; this applies well for example at the start of the game against the german or Italian fleet if you can corner it. In that example, the Commonwealth can afford to lose one ship against one German ship or one Italian: in the end, its anyway an allied victory. Against the Italians, using the French fleet is even best, since it will be lost anyway!
< Message edited by yvesp -- 12/27/2013 9:26:36 AM >