We're confident you'll get better results - in less time and with a lot less hassle. Ask to speak to the manager or the assistant manager at the paint store. Salesclerks probably don't know any more than you do, especially if they're new on the job. But it's likely the manager or assistant manager has been around paint for awhile. All paints aren't created equal, even products from the same manufacturer.
Every company turns out a variety of grades that run the quality gamut.
When you're shopping, ask the paint-store manager which manufacturer the better painting contractors use. Then ask for the premium-grade product from that manufacturer's line. Or, call the manufacturer to verify which of its products is best. You might end up spending twice as much as you would for a bargain brand. But you will notice a difference in coverage, wear and washability.