I've had the Braun entervan conversion of a Chrysler Town and Country for 7 years. I am about to buy a new van and have Braun convert it. The 1999 Chrysler has been excellent except for transmission troubles twice. I have 176,000 miles on it. I was thinking seriously about the new Honda and Toyota vans because of their reputation for better build quality. I looked at the Sienna and it was a nice van but it was too shallow behind the front seats. The Honda Odyssey was worse in some respects. The ramp is much narrower than on the Chrysler and it has a rubber sensor which is fragile and exposed on the edge of the door. I tore it with my scooter handle while going up the ramp. Once inside, because of the Honda suspension system, the lowered floor renovation is unable to lower a critical part of the floor near the two side doors. As a result you lose critical real estate for turning a wheelchair or, in my case loading scooters. I often take my outdoor scooter and indoor scooter, or a bike and a big scooter at the same time. There is no problem loading the two in the Chrysler. We tried it in the Honda and it is very difficult. The Sienna is out of the question. If you just have a small wheelchair and nothing else, the Japanese vans should be fine.
I would recommend the sideloader over the backloaders for convenience and safety in parking. As well, the wheelchair user is unable to drive or even sit with the other passengers.
Whatever van you get, it will improve your enjoyment of life immensely.