but with more miles you get the higher miles issues quicker :P I suggest low miles personaly, though if I was giving you real advice I'd say look for something better than a sienna. but if your set on it low miles is usualy a big win.
P.S. get a CarFax, independent inspection, & an extended warenty on whichever you get. check the oil and stuff too lol. just to be sure the dealer/independent seller didn't over look that