Be sure to read the whole order page, as many retailers have started implementing two-stage purchasing systems to stop bots from placing the order instead of a person. Best Buy, for example, has you click "Buy" once to join the queue, and then you must wait for the button to light up again to actually add the console to your cart.
Where possible, try to use a retailer's app to do your order rather than your browser. Apps are often times faster than the website, and for the best results, try using both simultaneously to double your chances.
Though both versions are prone to selling out quickly, the white Switch seems to be the more popular due to its sleek new look. During restocks, the version with red-and-blue Joy-Con controllers tends to stay in stock longer than its white counterpart. Its worth keeping in mind if you're only interested in the console's new specs and not the new look.