There is one notable difference that I've recently found: Air Bnb does not forward your money to the property owner UNTIL 24 hours after you check in. This gives a bit more security to Air bnb reservations, in my opinion, in case the listing is not as promised. We recently used both for six different properties on our European trip. One property owner, in Arles, (that we rented via vrbo) texted us the night before arrival, telling us that the toilet was "having a problem with flushing" Yikes! Upon arrival, nobody met us (turned out the homeowner was in Italy and her "friend" never showed up, even to take our money). The toilet was still operable by pushing the wall mounted flush mechanism into the wall carefully with each flush. In addition, the coffee maker was broken, and the tiny refrigerator was completely frozen over (we like our ice cubes!). My husband spent an hour defrosting and melting out the fridge. The texting owner kept promising repairs, but nobody ever showed up. Eventually, five days after we departed, she realized we owed her a 50% balance. We paid it, because the apartment was clean, beautiful, and in a great location with rooftop terrace, but I think if there was a go-between like Airbnb, perhaps we would have received a partial refund...
The service fee on vrbo varies from zero (those properties are also pushed to the bottom of the search list because they buy the cheapest possible listing) to ...I think it's 2%?
I would do as you're thinking, find the best apartment that meets your needs and TELEPHONE the property representative (if VRBO - Airbnb won't allow this because they don't want you to escape their fee). Get a feel for their integrity. I have almost been scammed twice on VRBO, but never on Airbnb. Ask lots of questions and you'll see whether they really know anything about the property. And it's much safer to rent a property with many reviews of course.
Have fun deciding!