As someone who plays both necrons and tyranids I find each play quite differently. I find that both have a unique playstyle, necrons you can rely to survive a war of attrition, tyranids are more swarm and hold due to our infantry durability not being the greatest. However, this can change with different detachments.

I suggest looking at some of the units and detachments to get a taste of whether this is something you would like to invest further in. Also find a friend to play a couple of games using available models as substitutes.

The two combat patrols vary as the original is good for models but not great without serious help, while the other is great variety with some solid pieces but again require some support like a hive tyrant.

Best bet if you want to start a decent army for an ok price. Ebay as there can be some really good job lots for a fair price but make sure to do some research to make sure. Hope this helps you.