mortgage advice - stick or swap?

I am in the process of moving to a new house. Had an offer for mine and an offer accepted on a house to move to.

Looking to borrow approx 150 against a purchase price of 200 so just inside 75% LTV.

Have an agreement in principle with our current lender - Nationwide but I am now tempted by some other lenders out there. Looking at possible Chelsea, britannia (co op) or leek united.

Would I be better going for a longer term fixed rate (5 yrs) or 2 yr?

By applying for others would it jeapordise the agreement with NW?

Thanks for any advice