if you can heath, go for the one you pay for. a chap in work has just done it. it was 10k but you get every penny back. it halfs if elec bill and he will get £1200 tax free back.
the one your on about has a few cons, for example if you need to work on the roof below the pannels, and they need removing you will have to pay for the energy they have lost. plus it may effect the sale of your house, as the new tennents will have to take it on.
ive also heard some morg companys wont give you one if you have that one