Which is better for a first time home buyer, conventional or FHA?
That depends on each individual. The FHA requires 3.5% down payment and the conventional loan is 10% down or more. Yes, with 20% down, you avoid paying MIP (mortgage insurance premium). However, there are programs that you can use to assist you with down payment. It depends on the County you're...

having 690/700 credit make a big difference?
RUN! Mortgage lenders are not in the business to repair credit. There are too many credit unions and banks who will finance you with a 695 score.