Which is preferable—loose or pre-packaged spinach?

You can get better quality with loose spinach because the format allows you to inspect all the leaves.
Select leaves that are smaller and have a good green color to them. Crisp, spongy leaves are especially desirable. Don't select leaves that are wilting, brown or yellow. Fresh spinach should smell sweet -- never sour or musty. Look for stems that are comparatively thin. Thick stems denote overgrown spinach, which may be leathery and bitter.
 
If only bagged spinach is available, check whether the contents seem resilient when you squeeze the bag.
 
Source(s):
Wikibooks.org "Cookbook: Spinach"