If you have a scale, you'll want to use do it by weight for consistency. Corn sugar can be fluffy. Table sugar can be very consistent if you use the same brand, so if you can get by measuring by volume if you find a certain amount that works well for you. However, if you switch brands from time to time, you can get different grain sizes that will pack differently so your volume measurement can change.
I wholeheartedly agree about using table sugar. It is cheap and easy to get. I target a certain CO2 level I want, then weigh out the amount of table sugar I need. I'll use a bag of corn sugar if I have it, such as if it came in a kit, but I'll still weigh out how much I'd want to use. To do the calculation, for simplicity I use Palmer's nomograph, but there are plenty of online calculators for that.
As for the differences in amount to use, table vs. corn, the math is that for every gram of sucrose you use, you generate 0.51 grams of CO2; for every gram of pure corn sugar you use, you generate 0.49 grams CO2. Most likely if you get your corn sugar from the homebrew shop, it will be glucose monohydrate, which has water bound to it to keep it from clumping. In that case, for every gram of that you use, you'd generate 0.44 grams CO2. The upshot to this is that if you are normally using corn sugar and you want to switch to table sugar, you should use something like 87-percent table sugar (0.44/0.51). This all assumes that the sugars are 100-percent fermented, which is a pretty good assumption, but you can see that when you move to other priming agents (DME, LME, brown sugar, molasses, etc.) you start to get more hand-wavy because now each of your grams of priming agent has more and more unknown unfermentables in them (water content, probably the biggest).
One reference for more geeky background on this is http://www.byo.com/stories/techniques/article/indices/21-carbonation/1276-priming-with-sugar
, or even better, http://www.homebrewersassociation.org/attachments/0000/2497/Math_in_Mash_SummerZym95.pdf