Think about what you're commemorating. Don't skimp. Buy the best you can afford, but of course buy smart. Put your eyes on what you're buying in person. Definitely try to haggle with them some.
I don't buy a lot of jewelry, but for my last big purchase I went to probably the most popular dedicated diamond retailer in my area where you're offered wine and champagne, and there's more plastic surgery in there than an army of Bruce Jenner clones. In short, I was disgusted how I was treated. A girl showed me a diamond that was clearly yellowish and tried to say it was "the lighting". Yeah, all those bright white lights in there made it yellow, and that one small stone was expensive as hell.
I ended up buying from Jared's where I got a fantastic quality piece (I'd done my research) for much less than what I would've paid at Diamonds Direct, and the sales dude was super helpful and chill and told me to wait until a certain date to purchase when a sale would be happening and saved me a ton of money. They seem to have a lot of sales because I still get crap in the mail from them. All the Jared's may not be like that, but I'll go back to mine next time. YMMV