What's an MP5? You mean the Mazda5 true-mini van? I like it, but it is small.
As someone who needs the seat to carry people, the Odyssey has way more space. If you use all the seats, there's next to no cargo space. (
pic) Mazda's website won't even list it. It offers 45 cu ft with the 3rd row folded. The Odyssey offers 38 cu ft with all seats up, 93 behind row 2. My grandmother has one and I borrowed it for a couple days. It'd work well for my car and occasional family duty, but as a primary family car, no.
The basic Odyssey you can get for 28 is just a van. Besides the Mazda, it's the best "driver's car" van. I don't need most of the junk that bloats the price to $45,000. That's why I'm glad the economy options are now available on the base model. To my mind, paying that much for "features" is just stupid.
To sum up, for $8k more you get a lot more vehicle and equal mileage. If you don't need the size, the Mazda is a good choice. If you do, I'm glad the Ody is there.