Here’s my two cents…
1. Is it important or valuable that the washer and dryer can stack on top of each other?
It depends on how much space is available. For my Airbnb, the W/D hookup is in a 38" wide closet, so I had no other choice than to get a vertical W/D ‘tower.’ They’re more expensive, but I was fortunate to find a vertical for half-price at a scratch-and-dent. If I had the space, I would’ve preferred a side-by-side W/D because they’re less expensive, easier to find as ‘used,’ and easier to replace.
2. How important is water usage?
Water usage is important, although the biggest impact I’ve seen with frequent washer use has been on the gas bill. I’ve alleviated some of this by lowering the temperature on the hot water heater.
3. Front-loading washers and dryers typically require some bending to load/unload. Is this an important factor?
So far, I haven’t heard any guests complain about the front-loader. A top-loader might also require some bending, depending on the guest’s height.
4. Front-loading washers are known to clean more thoroughly than to-loading washers.
Definitely a plus.
5. Front-loading washers are gentler on fabrics. How important is that to you?
I’d consider this less important because the guests are only there for a week or two at a time.
6. Front-loading washers and dryers are generally more expensive than top-loaders.
See #1.
7. How important is it that the units are quiet?
It depends on where the washer and dryer is located. For my Airbnb, the W/D closet is in the living room area, so I had to get something very quiet (LG Thinq) so the guests would be able to do laundry and watch TV at the same time.
8. Do you need a high capacity washer and dryer?
Yes, if you plan to use the same washer and dryer to wash the blankets/sheets in the suite after the guests leave, then a W/D with enough capacity to handle those larger items is essential.