I proposed to my (now) wife when I lived in Houston and used Whiteflash because they had such a great website, I could choose the diamond and setting, see realistic pictures, and they were local. But they actually ship worldwide and compete with the Blue Niles of the industry.
Once I had chosen what I wanted online, I went to their store in person and all it did was solidify the choices I had made (and help push me in a certain direction... see next paragraph). In other words, don't be afraid to buy from an online store; you could always visit local retailers to get a feel for size, but I suspect their pricing will be higher.
Since you are asking us this question I assume you are the kind to do research, which would imply you are aware of the four C's. My $0.02 after going through the process is to weight carat over clarity... I remember I had narrowed it down to two diamonds where one was higher clarity but smaller and the other was larger but a grade lower... my engineering mind wanted the higher quality diamond but I decided to go with the larger and am very glad I did. When they say you can't see imperfections without a microscope they aren't kidding!
To summarize, I would definitely recommend checking out Whiteflash website. They have amazing service and great pricing (at least when I bought 9 years ago). Once you narrow down the diamonds and settings, I would compare across multiple online websites to get a feel for best pricing, and be sure to consider the addl benefits/features of each company, for example, is it certified (i.e. you get paperwork certifying the four C's), conflict-free, etc.
Congrats and good luck!