Personally, I tend to err on the larger size. Because I read this, and I believed it. Riders that are more about performance and aero than comfort, would tend to err on the smaller size. The bike industry nowadays tries to fit everybody as if they are riding in the TdF.

My mountain bike is a motobecane 29er. I bought it in the 19" size, which says for 6'1"--6'3", even though I'm a hair under 6'. My wife (who is pretty tall, like 5'11") thought that bike was unbearably stretched out. But then I swapped the stem from 110mm down to 60mm for her to test, and now that's her bike. I bought the same bike in 21" (listed for 6'3" to 6'5"), and again swapped the stem 110-->60, and it's the same for me as before.

My commuter is a 60cm crosscheck, if that helps you at all.

So yeah, I recommend considering going with the larger size, but in the end it's your body and your preferences, and it's not possible to guarantee the right choice without a test ride. That's the biggest downfall of ordering bikes online. You can save a good chunk of change, but sizing is a risk. You should check if Nashbar will take a return/exchange if you change your mind on the size. Since they're the same company as Performance, maybe they can ship it to a storefront cheaper than shipping to you?