You can do quite a bit with speedlights, stands, umbrellas and other modifiers. For portability and light weight, they can't be beat.
If you are planning to be in a studio setting, strobes of some sort are more versatile and useful. But taking those on the road means you need to be sure of your power source or have a fairly hefty and heavy battery.
I'd start with a speedlight based system. The lightstands and umbrellas (and maybe some modifiers) will still be usable if you later go to studio strobes.
You also need to think about how you will control the lights. With speedlights, you can use teh Nikon CLS system very effectively indoors, and in some situations outdoors. Your D800 has teh commander mode built in so you ready to go. WIth studio lights or in many outdoor situations, you will need either cables or a radio control system. This is another expense to keep in mind.
By the way, I have both. I use speedlights for a lot of portable work, and have a good Alien Bee set-up with Cyber Commanders and a batery pack for studio work or for bigger jobs.