Unless you’re set on David Guetta, bands will usually cost more than a DJ, so if you’re on a budget, a DJ or playlist will be the way to go.
That said, with the range of live acts available, budget doesn’t have to be the deciding factor. Some three-piece bands, especially rock and indie bands where the guitarist or bassist is also the lead singer, can still create a huge sound, while not being too much more expensive than a DJ.
For those on very tight budgets, an iPod/iPhone playlist might seem like the obvious solution. But be warned: you'll likely still need to hire in a PA system to plug this into. Even if your venue does have an in-house system, these are usually designed for background music and the speakers will likely be small and tinny.
If hiring a PA, go for at least 500w per side with bass woofers, or it's unlikely your guests will feel moved to dance.