If you are into racing every gram can count. Otherwise get the heavier/cheaper tubes!
70 grams is like 2 1/2 ounces and every ounce of rotating weight (like on your wheels) is the equivalent of taking twice that amount (or 5 ounces in this case) off the weight of your frame.
Does that really matter to you? Is it cost effective? Will you be faster?
Hey, you control the purse strings; you decide!