Deciding on what kind of antenna you need depends on how you are using EVDO. Are you mobile or in a fixed location?
If you are mobile a Omni antenna will work well because it radiates its energy in a 360 degree pattern:
Since you don't know where the towers are, and are being handed off from cell to cell, this is an efficient way to get a signal. Its not so good for a fixed location because all that energy is lost, when it could be directed toward a single cell tower.
In a fixed location a directional antenna will obtain a better signal:
Notice how all three of the above antennas direct all of their energy in one direction. That is going to give you a stronger signal with less noise than an omni. Even without Line of Sight (LOS) these perform much better in a fixed location .
With a omni you just put it up as high as you can and that's it. With these directional types you have to aim them. Knowing where the cell tower is and observing your signal strength meter are important tools to achieve the optimal results.
When ordering one keep in mind the dB rating. A greater number will give you longer range but also narrow the beam. Aiming becomes much more critical.
Other posts I have on this blog show you the type of connectors and cable you need to put this together. It may cost a little more initially but is well worth it to obtain a good signal and speeds.
This is my setup, easy to install, and gets a great signal:
If fixed, you aren't moving and neither is the tower. Use a directional antenna, save the omni for the road.