I have a SB900 and a SB700 flash and was wondering if I should learn studio shooting with the or purchase some a studio flash kit. With speedlights is it worth just buying umbrella and stand for them. Any help would be great.
Speedlights can do many jobs satisfactorily. An excellent starting point if you already have them (seriously, go for it). They work well in umbrellas. See
Mounting Speedights in Umbrellas
Yes, there are differences (mostly of convenience), but there is no reason not to start there, get your feet wet inexpensively. Anything you learn there is universally useful in any later situation. Any light stands and umbrellas can be used with studio lights later, the speedlights can even serve as extra lights. It will not be a waste. I prefer optical slave triggers for studio situations (your SB-900 and SB-700 already have them, and studio lights all have them), but if you buy radio triggers, get some that work with sync cords or hot shoes (or hot shoe adapters), then no waste either way.
The main differences (with actual studio lights) are:
Ease and possibility of adding various modifiers (softboxes, grids, snoots, beauty dishes, whatever). Umbrellas work fine on speedlights, but the rest is a kludge. There is no mounting capability, and you cannot remove the speedlight fresnel lens in softboxes.
Maximum power capability. You don't need a lot, but a little more is a good thing.
Recycle speed (a biggie, waiting on next shot when subject is peaking)
Duty cycle (frequent repetitive flashes heating it up - studio lights have fans)
Speedlights can have some advantages...
TTL capability with the Commander (fast setup in simple situations, however then system expansion and versatility is rather limited)
Portability, runs on AA batteries
See
Speedlights vs Studio Lights