I don't think the Mazda 5's gas mileage is much better than the sienna. I own a 2003 Mazda MPV and when the 5's came out last year I researched them.
They get about 28 mpg highway, which is the same as my MPV gets but the 5 is MUCH smaller. I mean TEENY. It's awesomely cute looking and very fun to drive, but you're going to feel the smallness.
And I felt the difference from my MPV, which is already a substantially smaller minivan than the Sienna.
Also, conventional wisdom says to never buy a new model car the first year it comes out because they haven't worked the bugs out yet. THis was especially true for the Nissan Quest minivan-it was notoriously buggy the first year. I'd give the 5 a year or two.
As for reliability, my Mazda's been terrific, knock on wood. My friend with the same age Sienna just spent 700$ having the gaskets on the transmission replaced, and her rear liftgate stopped working. My husband who drove up to kentucky in a rented 2007 Sienna yesterday had to stop and trade it in for another one at an airport because after an hour on the highway the check engine light came on and it started smoking. He swapped it for another Sienna, this one a 2008, and guess what, three hours later, light came on! No smoke this time so they kept going, but he says he was really surprised that a Toyota would do that.
We are big toyota fans and have owned several, but probably won't ever buy a Sienna because of what we've experienced and what our friends have experienced.