It really depends on what you are looking for. If you use them for bright, vivid images, then go for the Inktense, then start a small collection of water colors pencils. ( And I must agree with Koffee, DON'T get the Faber Castells. I found some in my painting bag that I must have bought for field work??IDK but I don't like them AT ALL. I let my kids use them LOL)
Now, if you like what has the potential to be softer more subded colors but has the potential to be made vivid and vibrant ( through layering and glazing colors) then you will want to get the water color pencils and start a small collection of the Inktense seperately. They appear to be the same thing, but once you use them they really aren't. Inktense can't be lifted out, or blended once you put them on the paper..but they are wonderful to have. Watercolros can be mixed and blended and lifted...and there are some colors that are permanent but not all.
Personally I say get the one you will use the most, then start buying the others. It is difficult to fill in a water color collection with Inktense.and visa versa..you can't really fill in Inktense with water colors and they are not really the same. IMO. They aren't exactly compatible unless you use some of the deeper inktense to shade and or highlight your water colors..they are too deep and permanent. Equally, IMO, you can't lay water colors over top on Inktense because water colors are essentially transparent..Does that make sense?
Like I said..get what works for your individual coloring style..and once you are good there..then start working on getting the other..Happy coloring!