[Baalz]

While (as with most questions of this type) it will vary depending on your specific situation, kitting out a SC properly is almost always more cost effective- particularly in MP where your opponent will be actively trying to kill your best dudes. There are very few creatures who can singlehandedly take out an army straight out the box, and very few that are any threat to a kitted SC without equipment of their own. So if you're not killing armies, and you're not killing SCs then you're much less than half as valuable as someone who is. I can't think of any combat-use summon I'd rather have two of than just one properly equipped.

[edit] Ok, I did think of a case that you will often want more rather than properly outfitted ones - immortal units. Since the equipment is lost if they die, leaving them naked allows them to be used completely disposable. This still isn't a no-brainer, as anybody who's fought kitted wraith lords knows, but LA Ulm's vampire counts, for instance, would be wasteful to equip with anything.