Agreed that DD is generally a better buy. 24 IPC either gets you :
2 cruisers - 6 attack, 6 defend (3,3 after 1 is a casualty and 0,0 after 2 are casualties) after 2 casualties, it costs 24IPC to get back to original strength.
3 destroyers - 6 attack, 6 defend (4,4 after 1 is a casualty and 2,2 after 2 are casualties) after 2 casualties, it costs 16IPC to get back to original strength.
The only time you should buy cruisers is:
-You have a specific bombardment need, or
-You are limited in the amount of units you may place at your IC, or
-You have no cruisers in the seazone and you want your navy to look bigger than it really is.