We have been in our 2014 Sienna Limited for just about 4 years, overall vehicle has been very reliable...that said the technology and infotainment system is beyond painful, I know they have updated it since, but even the new stuff is no where near U-Connect...also disappointed with Toyota for a couple of issues we had, in the 2014 the 3rd Row squeaks and rattles something awful, there was a recall notice in 2015 with more info and a fix to come, here we are over 2 years later and still no fix and the noise is very annoying...
That said the all wheel drive has been fantastic, but does not apply to the model you are looking at...and finally our 2018 Pacifica is built and on its way to us now and will be trading off our Sienna, as many other have said the Pacifica is just light years ahead in many ways. Both are obviously good vehicles, either is a good choice, but we chose Pacifica.
Go drive both, most find it an easy decision after driving them. I just hope the quality and reliability is there in the Pacifica, I am currently having a few issues with my 2017 Ram 1500, after coming off a flawless 2014 Ram, so fingers crossed.