Hi James Just read through this thread with interest - it is a tricky decision to make and it is difficult to compare. It seems that all Caterhams have their own unique attributes with no two being the same! Also so many variables with respect to gearboxes, diff ratios etc which change the way it feels/drives! I have a Sigma SV with just under 200bhp with a longer 1st and 2nd in my type 9 box and love it. It revs freely and sounds great! I have not driven a Duratec and mine back to back to really appreciate the differences, but largely agree with the comments expressed! Sounds like it will be great to build it with your dad and you will obviously enjoy it irrespective of which car you build. For me I would think hard about what you want from your car and then look for the best fit. If you go Sigma you must speak to Stuart at Premier Power. If you want brutal torque, then consider the Ecoboost for Duratec money or go for the Duratec for less torque but with different characteristics to the Ecoboost! Wanted to say more but family matters drag me away! Drop me a line if you want to talk further! Cheers Garth