alphard is top car of toyota. all the best technologies are applied on this model. it won't be a wrong decision to purchase alphard as your 7 seaters. honda new odssney is a good substitute. if i were you, i chose alphard.
Prairie - 內籠比較 small, besides my friend (專買賣車的) reminded me the engine reported failure frequently for those older than 3 years but I like the opening of small window at the back!
Stream - Engine is good and interior is comfortable
Picnic - Overall is good but the 2nd hand price is quite high
Previa - Big interior but expensive
Odyssey - Overall is good, interior is comfortable and not too high (方便入停車場)
I finally bought Odyssey 2001 for $13x,xxx!
BTW, 電 slide door is convenience however they usually 太慢 and easy to 壞 for about 3 years usage.