I did recently downloaded Baby Monitor by Annie from Apple store and it works great! Don't really need to buy the expensive monitoring devices as with two children, I don't have much money to waste really. Of course it's for the safety of the children, so it's not really a waste, although I can easily use this app and it does everything what the big baby monitors do and even more. I've just used my old ipad as the second device placed near my LO.
It really depends on what you feel most comfortable with. There are audio monitors that allow you to listen to any noise coming from the nursery, vital monitors that track sleep and breathing and video monitors that add sight to sound. Babylist parents overwhelmingly choose video monitors. The security of seeing what your child is up to—like if they’ve gotten tangled in their swaddle, pulled their diaper off or climbed out of the crib—can be worth the extra cost of a video monitor.