Putting the tesla or gauss weapons on the blades is, in my opinion, a massive waste. Why get a flying immortal when you can just get two immortals? Necrons don't do MSU very well, and operate best in FLU. Get the unit of ten immortals over the five blades.
However, immortals can't take particle beamers (Oh god, if they could...) while blades can. Tomb blades are highly mobile, and will simply kite any enemy the opponent tries to throw at them. Against hordes, they are amazing. You want to have imotekh so they can avoid the worst shooting (AP3 or better weapons tend to be fairly scarce, and longer range than others), and give them shieldvanes to force the enemy to prioritize their fairly less common AP3 firepower their way. The downside? Five models. This is why you need Imotekh, so they can't be focused to death to deny RP. There is really very little that can take advantage of the night fighting better than tomb blades can. High mobility with a 24" range and the ability to quickly zip to a flank is great. Even things like Wraiths still have to hope for the best when using the 24" range, as they will almost always get shot at while moving forward.
On the other hand, Deepstriking wraiths behind terrain is a really nasty trick. The enemy will want to get rid of them before they simply jump over the terrain and charge something. They are big, killy, and scary, and supremely hard to kill. However, S8 instant death (Hammernators/PFists, anyone?) is a threat, as is being shot to death.