There are many places you may choose to buy popular windows such as Simonton. They are sold and installed by the big box stores like Home Depot, and also from contractors in your area. Pricing typically can be higher from either, so it’s best to receive at least 3 quotes from both places (Home Depot and local contractors). Warranties for the product will be the same across (as should be the labor warranty). The difference between big box stores and contractors is how long they’ve been in business, so they can uphold their labor warranty and the reviews.
The determining factors are finding the right window best for your home and needs, the reputation of the contractor and how long they’ve been in business, and lastly who you feel most comfortable with.