Think of the Suica card as a prepaid train card, where you can load a specific amount , the minimum would be 1000yen. Now, minimum fare for Metro is 150yen, so you can used be metro as long as you have a load. If you are only using the Metro once in a day then you only need to spend 150 yen.
On the other hand the day passes is fixed around 790 yen . By the time you used it, that pass is valid on that day until the last train. Think of it as a ride all you can on the Metro for a day without worrying wether you still have a load or not.
So it will now depend upon the frequency you will use the Metro, and u also have your JR passes.
I am not sure about Suica. it does not really matter just pay in cash its all the same anyway ... suica or not..
