As KenH says, if you can, upgrade the double to a queen. We are still sleeping in a double, but often it’s not ideal .
On the sofa bed: Yes, me and my mum would prefer your place. And like you indicated yourself, there are more people traveling together who would prefer not to sleep in the same bed.
If you would consider allowing more than 2 people, 3 or 4, a sofa bed will also allow you to open up to that market: Couples with children, 3 adults, etc. … Since a large portion of listings are only made for couples or two people who are willing to share a bed, there’s a whole other market still to be catered for.
Do think about extra storage, chairs, …
On the quality of the sofa bed. In Porto we have an IKEA sofa bed. The following but with a double bed:
For people that are not too heavy (=> make sure to put a BMI limit in your houserules = joke regarding other topic), this sofa bed is comfortable enough to sleep on for a few days. If you advertise it as a sofa bed, everyone will know they don’t have to expect a luxurious boxspring to sleep on.
If you have enough space, you can also consider designing your own sofa bed with two mattresses (e.a.190x80cm / 200x90cm). That way, sleeping comfort is guaranteed. To give a random example http://littlegreenbow.com/2012/01/detailed-diy-couch-plans/.