Quick answer:
Queen size affordable memory foam pillows
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Why memory foam pillows for my Vacation Rental?
Memory foam looks better and lasts longer. Every pillow goes flat eventually but memory foam (especially with latex stuffing) will stay plump longer. So that means fewer replacements you need to buy. Even after plenty of use, a memory foam pillow maintains their full look so your pillows will look great for guests and comfy in listing pics.
Why should I use queen size pillows for my Vacation Rental?
It is the perfect middle ground between price, comfort, and flexibility. A queen pillow is about 20 x 30 inches, a standard pillow is only 2 inches shorter, but a king size pillow is 6 inches bigger. So a queen size pillow will pair fine with almost any pillowcase but a king only works with a king-sized pillowcase. So I recommend only using queen pillows on all your beds so it's no problem when they get mixed up. Making your hosting life a little easier.
King sized bedding is also more expensive. If you have a premium vacation then go with the extra expense, but for most rentals, the more affordable queen is the better option. Now you can go smaller than a queen but you be sacrificing guest comfort. Most guests are used to the feel of a queen pillow, so even two inches smaller and something just won't feel right. So go with the queen pillow! It keeps guests happy, saves you money, and works on all your beds.
Why should I NOT spend allot on pillows for my Vacation Rental Pillows?
It's simply not worth the extra money. You want good quality pillows so guests are comfortable, but after a certain threshold, guests will not be able to tell the difference. The subtle differences of a high-end pillow are usually only noticed after someone has slept on the same pillow for weeks at a time (IE not your guests). Save yourself some money with a solid mid-priced pillow. Pillows go flat, liquids gets spilled, or sometimes they disappear so make sure you have a reliable source of affordable pillows to save you time and money.