If it's above a certain clarity/color (typically >VS2 and >G if I remember from way too much research last year) , it's virtually impossible to tell the difference with the naked eye - not sure I'd splurge there. I'd probably go for the extra carat weight.
Also, a rec on what I did was go to a couple local stores, find stones I really liked, shop online for the exact same specs, and then email those prices as counter offers. One store wouldn't come close to matching the price while the other actually beat the online price by a bit, and offered free resizing, cleaning, etc. Saved me a lot of money for the exact same stone I would have bought.