Depends in the wig and how long you plan to wear it. For something quick and fun I usually buy synthetic. But for classic styles I plan to keep in my arsenal to wear occassionally I typically buy human.
Synthetics tend to hold their style longer. If it rains, gets wet, humidity, etc. The curls won't fall. They're also inexpensive. You can find a decent synthetic for about $20.00. But whatever style the wig is initially set in is the way it will stay. They have a tendency to look shiny so make sure you try it on in the store to see how you like it.
Humans behave like real human hair. You can curl, wrap, wash, style, etc. But if you get it wet, go out in the rain, etc. the curls will fall, With human hair you may have to flat iron, curl, bump, etc. for the wig to maintain it's original style. And sometimes after you wash it, it may never look the same again.
Human doesn't neccessarily last longer than synthetic. It depends how often you plan to wear the wig. HH wigs also shed which will cause the wig to thin out over time. Once it thins out, it may not look good anymore. The quality of the human hair on these wigs usually isn't that great either so it may begin to look ratty after a few washes or after wearing it for an extended period of time.
Also human wigs are usually priced by the length. If you are looking for a wig past say shoulder length expect to spend probably at least $75 for a full wig. A half wig may be a little cheaper.