Unless you are 100% sure that you'll get approval, use a broker.
In my recent experience, there are many parameters the banks use to rule the applicant out. If you get turned down, you've got this mortgage refusal on your record. This black mark is a further negative which may go against you in the future. A mortgage broker will be able to flag such a pitfall in advance, and allow you to sort it out before you apply for the mortgage.
My saving record over the past 12 months had been excellent, I thought. I was putting away about €250 per month, every month. Apart from December, when I did not put away that €250, and actually dipped into my savings to the tune of €100 (approx). When my mortgage broker saw this he advised postponing any application 'til July at the earliest. Apparently I should try to have a run of six months, good, positive saving. That little Christmas spurge of mine, which I thought was innocuous relative to my otherwise clean records, would be enough to sink this application, and potentially sink future applications due to my "black mark".