The issue of "since they're an outside lender, and if they don't close on time, you'll get charged $300 per day" isn't based in reality. Yes, if you delay closing you could get charged. Yes, if your lender really screws up, you could get charged, but in both cases the vested interest by these parties is the same - close!
Trouble arises when the HOA can't get their act together or there are appraisal / insurance conflicts. Those are outside of the buyer, the builder, and the lenders control. I had a recent Stonegate closing that took 2,000 pages of HOA documentation to get just to straighten out the HOA's responsibility for future phases of a Condo project. Getting that level of documentation was terrible and the management company really was less than helpful. The insurance company for the Sub-HOA was in control of future phases, but didn't have copies of the builders insurance that covered those dwellings. That took another week of arm twisting to get. Even with a mountain of paperwork and so many people to deal with on important legal issues, the loan still closed on time. It's an example of how competence can make or break one of the most important financial decision you can make. When it's a late close because of the HOA, Insurance, or the Builder, you won't get dinged and they often turn out to be the problem, not the solution.
To me, the bigger part of deciding who to work with does boil down to competence. The builder uses certain lenders because they are market competitive, know the project, and more than often close on time. In doing your research with outside lenders (TI, Zillow Mortgage Marketplace, Lending Tree, Yelp) you'll find out pretty quickly who knows what they're doing and who's just slinging nonsense. The competence question is answered pretty quickly when you ask the right questions.
The second part - costs and rates - can't be answered right away by any lender- builder preferred our outside lender. Comparing rates and fees in a 4-5 month build time is a real waste of your resources so my advice is don't worry about solving the fee/rate puzzle at this stage. Roughly 65 days out, you get in writing on the same day a fee sheet (not a GFE) from 1-3 lenders, find one trusted mortgage professional out of all of them to help interpret the sheets as they could still be BS, and make your decision then and there.
A board member here on TI who was purchasing new construction had taken this advice, got a couple of quotes, and decided to use a major bank since they had the best price and, more importantly, they could commit to close on time. This decision was made 60 days prior to getting their keys to the home, and they got a fair, market competitive deal from a reputable lender. I'm sure once the decision was made some relief was felt since the hassle of deciding who to close with was now behind them.
There's a great deal of slack in the mortgage industry today and you'll find people quoting things that just are bait to bring in something---ANYTHING so they don't close the doors next month. I strongly believe you'll avoid that kind of problem, along with other issues that come up when buying new construction, by taking my advice on how to pick who you'll work with.
My .02c
