I'm not sure about NetInstall but Altiris has much more to offer than Ghost. Where I work we've chosen to go from Ghost to Altiris because Altiris replaces 3-4 of our current applications. It is also much easier to use. As far as these 2 software deployment/imaging systems I would stick with Altiris and just buy the updated packages.
I'll be interested to see what people have to say about NetInstall...