You can buy interchangable handles with a locking system that will work for flat bands and tubes, as well as several combined together.
I personally enjoy the flat more. I have tubes from 10 lbs of resistance up tp 120 lbs and extra light to ultra heavy flat bands. The flat feels nicer like the coach said on certain exercises where you are pressing them to the body. The also store easier in small ziplock bags if need be.
The only down side I have found for the flat bands is that they have to be replaced more often. I don't know why, but the resistance doesn't last as long (wear out faster).
My advice, invest in a high quality flat band. They last longer and have a little more "umph" to them compared to the cheapies you can get at walmart.
a good site to buy them, although a little pricey, they are my favorite even compared to the medical quality I get in physical therapy: shop.cathe.com/TPE_Non_Latex_Stretch_Bands
This site also sells Fire Walker Bands which go around the ankles for lower body work (circle band loops); I love those too!