It depends on various factors
1) If you take up the builders mortgage it usually cheaper.
2) Interest only is higher and not many Banks are offering.
3) What % are you borrowing
4) If you take Banks other products e.g credit card, Insurance etc.
Shop around and negotiate hard, the Banks/Branches have to meet their targets. I hope it helps and finally, if you take the builders mortgage you will save cost on mortgage transfer etc.
Good luck