According to informed for life, the ratings on those two vehicles aren't significantly different. If that's what's available in your area within your budget, I personally wouldn't worry about your son that much. Having him properly secured in the backseat in a rear facing car seat will make him very safe, probably the safest person in the whole vehicle, and if he's your only child you can put him in the center seat which has an even greater safety advantage. One thing I will say though is that the Toyota will have rear head rests and the Buick does not, that's extremely important for any adult passengers you may carry in the back and, if you keep the vehicle long enough, you'll need those when your son eventually graduates to a backless booster seat. In my experience Toyotas are also more reliable. I'd weigh those things against price, mileage, and vehicle condition to come to a decision.
ETA: here's the informed for life link
http://www.informedforlife.org/demos/FCKeditor/UserFiles/File/MasterSCORE2010nov10.pdf
And graphics showing stats for both the Toyota and Buick.
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