With the coming release of the Adeptus Mechanicus/Skitarii or the Skits as I prefer to call them.
There obviously has been leaked info and photos of their stat line as of right now they are superior to Tempestus Scions +1 ld, cheaper point wise, weapons look superior on paper.
images exceed posting size so I have to link them.
http://whec-assets-live.s3.amazonaws.co ... 69cb8f.jpg
http://4.bp.blogspot.com/-Ycc4khMM4Ug/V ... 0/ad_7.jpg
Also on GW own Website the Vanguard/Rangers are being sold for £23.50 for 10 models Tempestus Scions are £21 for 5 models. Yes you can turn the Scions into a HQ or troop choice but honestly can you really say that is worth it for almost double the price of an army which is arguably stronger?
I hope no one recently bought the Scions as a separate army or in mass.