Hi, great question. Its a tough one, I stayed at both last October and both are fantastic. Lukula just edges out Sand Rivers in my book though. I love really remote places and Lukula is a smaller more intimate experience in a classic tented camp in the middle of a huge area of wilderness. You literally have tens of thousands of acres to explore by yourself, no roads, no other people, just you and the wilderness.
The managers, Mark and Chloe are also two of the friendliest people you're ever likely to meet and Mark is one of the best guides I've ever come across in Africa (and I've met my share)
If you have time to see both then all the better but if you're limited to 3 nights then I'd go with Lukula. It's kind of like seeing multiple camps in the Okavango Delta, one can be enough but more than one just adds that bit of perspective.
Hope this helps.
P.S. You should definitely do fly camping there too!