Because of his diverse skill set, Neji should be able to master both the Rasengan and the Chidori.

Rasengan requires the user to not only have the chakra level capable of forcing chakra on the hand but the chakra control to properly focus it. As a prodigy of the Hyuga, Neji has chakra control superior to Naruto. If anything, Neji should be able to learn and master Rasengan more effiently than any character we've encountered so far.

As we all well know, Chidori is a technique unique to Sauske and Kakashi. It requires large stores of chakra, the ability to harness and control concetrated chakra, and an eye jutsu to see defensive counters. Neji has all of these. His Byakugan is as effective as Sharringan in its seeing abilities. There is no reason that Neji can't be as good as Suaske with the Chidori.