Hold on a mo, are you comparing like for like there (are they both bonus accelerator policies?). Be very wary on the accelerator policy, other insurance firms won't recognise it as No claims bonus. It's a bit of a con really as whilst it sounds attractive not only are you actually missing out on 2 months of insurance, but also when it comes to renewal time they hike the premium up a bit knowing that you can't go anywhere else as you don't actually have a full years NCB.

I personally stick to the 12month policies, making sure all excesses are the same and then compare companies. It's the easiest, safest and IMO best way to do it.

Good luck.