go with C'tan man... also id say ditch the NB. he is way too many pts for only 1500. for the cost of JUST the NB, you can field 20 warriors. also the warriors will prevent you from phasing out... the NB wont. there is a very famous anti necron saying:
"in games under 2000 pts, necrons will always fall due to the necrons using a C'tan." not many people ive seen actually USE C'tan, but the whole point is that if your playing 1500, even the deciever would be 1/5 of your army. as i said before, the NB might help you win in certain situations, but more warriors will help you not lose. and there IS a big difference between the two.