I wanted to like the Spark but I couldn't. It weighs only a little less than the Sonic but has a much smaller motor: 87hp vs 138hp, and as a result it has no pickup for merging or even going up a steep hill. Gas mileage is also unimpressive, you might expect the tradeoff of a smaller car and smaller engine would be more mpg but this isn't the case with the Spark.
I don't normally argue with Swordsmith as he's usually right, however, this time I have to disagree. I owned a Sonic LTZ up until this weekend and my mom currently owns a Spark.
I don't know what was going on with Swordsmith but my mom's Spark gets excellent mpg. Most of her driving is in town with about 5-10% of it at highway speeds and she averages 40 mpg on her last 3 tanks. Also I recently drove her Spark for a day while my Cruze was in the tint shop and it has excellent acceleration at town speeds (I didn't have the opportunity to drive the highway).
Obviously everyone's experience will be different but her Spark is an excellent car for in town.
Also, Money will be a major factor so just buy the one you like better and can afford.
P.S. While my Sonic had a bit more room in the driver's seat than the Spark I was still fairly comfortable in the Spark. I am 6'4" 300lbs. btw.