Neither really. At this point you should be getting pre-approved. Have them run your credit and provide them with your paystubs, bank statements, tax returns, etc.
Everything will need to be updated within 30 days or so of closing anyway, and you generally can't lock in an interest rate until the home is closer to being complete...normally 30-60 days out, Although some lenders offer "extended rate lock" programs for 6 months, you need to realize you are not locking in at today's rates that you see all over. It will be higher. You pay a premium the longer you lock in for.
So really your final application for the mortgage is probably going to be when the home is about 30-45 days from being ready to move in to. In between now and then you should focus on increasing or maintaining your credit scores, saving up as much money as you can, and not doing anything that will negatively impact your credit - such as opening up any new credit/store cards or taking out any new loans.
I've done hundreds of mortgages for people building new homes. If I can be of any assistance to you throughout this process, please do not hesitate to contact me.
Tony Grech | Mortgage Loan Originator | NMLS #977416
PMAC Lending Services, Inc.
Toll-free (855) 642-4762 ext. 278
Mobile (313) 622-7383
Fax (248) 945-4842