Get the Nor card Silver, its for 25 AED and comes pre loaded with i think 14 AED credit, i got the card from my cousin as he had an extra one and i ended up getting it loaded with credit, benefit of the card is that you can use it on the metro as well as on the bus. Download the app called WOJHATI on your phone it will become a life saver, it tells you closest metro/bus station and upcoming bus times among other things, its location based and will help you a lot.
Just avoid 5-6 pm and early morning time for metro as the freaking thing is jam packed