For fast attack, you cannot beat the Vendetta. Outflanking with 3 twin linked lascannons. With the transport capacity, it make for a very effcient way to drop melta or plasma vets on a back line to threaten enemy vehicles.
As to Straken, I definitely support taking him, though I field him with Nork, two melta's and a medic. In carpace armor it is a mean combo. Nork being toughness 5 means you have something that you can put s8 and s9 wounds on without losing an entire model. His ripper gun is also mean. And on the charge, he is s6 and so can do some damage to light armor as well.
The Manticore is a great weapon to use against clusters of troops or vehicles. Always look for clumps. Try to find someplace that mean you will hit something no matter how it scatters. I have managed to pop three rhinos with one missile once. Doesn't happen often. but when it does, you opponent tend to just look in horror at the wreckage you just incurred on his line.