What does the word eternity make you think of? Something eternal, never-ending, uninterrupted. Accordingly, an eternity band is a ring where the design or gemstones cover the entire surface, uninterrupted. This allows your ring to spin on your finger and always appear the same. While appearance is the strongest reason to choose an eternity band, there are other aspects to consider when choosing an eternity ring.
If you expect your weight to fluctuate in the future, you may opt to have a small sizing bar in your ring. A sizing bar of a few millimeters ensures that sizing your ring up or down in the future will be easier, and thus a less expensive repair for your jeweler to perform. I love the look of an eternity band, but I recommend a small sizing bar to ensure that potential size changes in the future are easy for your jeweler.
Having diamonds go 1/2 or 3/4 of the way around your ring will be less expensive than a full eternity band. But, if you decide a couple years down the road that you do truly want an eternity band, that repair may be more expensive than just springing for an eternity style in the first place.
Here's another tip: If you want to make sure your wedding band doesn't spin, you can ask your jeweler to solder your engagement ring and wedding band together (assuming you intend to wear them both everyday, of course!).