Like Icculus said, the burna/loota box is great value
IMO. All you need is a few extra Ork torsos and legs kicking around (which any self-respecting Ork player ought to

) and you can easily build four of each from every box.
The nob box is awesome, so many extra parts to use with the rest of the army. They're great looking models too. I tend to run my nobs like a crazy person - a big ass unit of 10 in a battlewagon to go with my warboss - but if they survive their way across the table they absolutely wreck face.