First of all, bear in mind that the rules are only experimental currently.
http://www.forgeworld.co.uk/Downloads/Product/PDF/c/caestus.pdf Cestus and Stormraven cost about the same. (cestus is a little more)
Cestus has
AV 13,13,11 - so better on front and sides but worse on the back. That rear AV12 on the stormraven makes it VERY strong when being assaulted.
Cestus can carry 10 termies, more than the SR. The SR carries more
PA and a dread. SR can also carry jump packers.
Cestus is faster and has a special ramming ability.
SR has way more guns but the one gun the Cestus has is a large blast melta weapon. That weapon is mounted in a position where it can only fire dead ahead - the storm raven has it's weapons on fairly good mountings.
The Cestus is a fast attack choice while the SR is a heavy support choice. For the kind of armies that would want a vehicle like this, the
FA choice is competing with other stuff you would want.
Overall, I think the Cestus is a better troop delivery system - especially for termies - but the StormRaven is a better all-rounder.
I wouldn't say it's overpowered.