The disadvantage of having more power is NOT having it when you need it. If the size of the units cause them not to be available when you need them - its pointless having them.
If you're wanting to use these predominantly in the studio, then having more power is especially useful with large modifiers or when you need to light larger groups or sets at increased distances. If you intend using these outdoors, then every increase in output that can be attained would be more than welcome.
I use both the 500's and 1000's together and have never experienced any disadvantage with either.. but I do like to match outputs where I can - kits of 400's, 500's, 1000's etc.. are always easier to work with. I think a couple of 1000's would be quite useful.
If you shoot products using smallish softboxes you will probably find the 1000's too powerful. If you're shooting full length portraits with umbrellas or larger softboxes then you will be well suited.
Theres only one sun. Why do I need more than one light to get a natural result?