The 6 speed is a rock. The 8 speed is a little new but so far it’s doing ok. If mileage is about the same I’d go with the 2016 personally. I will say though Toyota is really good about recalls. They do theirs voluntarily where some manufacturers fight em. 

Test drive both to see what you like better. 

I personally would stick to the 2016, because of the transmission. 

The Aisin 6 speed is better than the Aisin 8 speed, but even the Aisin 8 speed is very good overall so you can't go wrong either way. 

The 2016 has 30 less hp and gets worst fuel economy. Think about it. Some people buy Chrysler pacificas, so I don't think a sienna could be worse than one of those. But, I would get the 2016 sienna, because vans were never known for speed anyway, or resale value for that matter. 

Get the 2016 w/ 6 speed automatic

For longevity, I would go with the 2016 Sienna since it uses the 6-speed AT.  It has 2 less gears (and complexity) than the 2017 (which uses the 8-speed AT).  But overall, either model year Sienna is still better to get compared to other minivans on the market.

I would get the 2016 Toyota Sienna with the 6 speed automatic. Those have been perfected for many years. Also, the 8 speed automatic hasn't been proven to be reliable as the 6 speed. I would say that the 8 speed reliability is a little bit behind than the 6 speed automatic.