you'll get better prices (even while using a Groupon)
Like what?
Say you buy the 16x20 Groupon - what you get is a coupon code for $127 - the price of the print and shipping. Since the pro price of a 16x20 is $100 shipped, the Groupon actually gives you a credit of only $100. However, now you could upgrade to a bigger size, say 20x24 for only about $15, instead of the usual $25. This price difference gets considerably bigger as the prints get larger.
Of course if you want a 20x24 you should just get the Groupon for it (which is only $7 more than the 16x20), but if you want something bigger than what Groupon offers you'll save a fair amount.