I’m actually going to answer the question with a wider focus than just the 5D or dSLRs, but for interchangeable-lens system cameras in general, to try and get a more useful answer.
When it comes to geotagging your images with a system camera, there are several options, some rely on the camera feature set, and some don’t. I’m listing them from least to most effort (and possibly most to least cost, assuming you have a smartphone).
1. Get a camera that geotags.
Some cameras have geotagging and GPS capability built-in. For example, swapping from a 5D to a 6D would gain you this capability, and you’d just have to switch it on in the camera. This is by far the most convenient path to take, with the main caveats of reduced battery life and a more expensive camera.
2. Get an OEM add-on for GPS.
Nikon and Canon both make GPS devices (Nikon GP-1, Canon GP-E2) that can be connected to some of their camera bodies, and relay the GPS information so that geotags can be added to the EXIF as you shoot. They may not be the best GPS receivers on the planet, but they speak directly to the camera so you don’t have to do the geotagging in post.
3. Get a camera with wi-fi and a smartphone app that geotags.
A number of system cameras these days come with wi-fi capability and the ability to communicate with an OEM smartphone app. Many of these apps can also use the GPS/location functionality of the phone to add geotagging data to the camera EXIF. Canon’s app, however only seems to do this with Powershots, not their dSLRs. And the Nikon and Sony apps have no geotagging function. But the Olympus, Panasonic, and Fuji apps can add geotagging with compatible camera bodies. You pair the phone with the Wi-Fi network of the camera, turn on locationing, and go shoot–there may be a syncing step to add the tagging. Again, battery life of the camera and the phone are probably your main concerns.
4. Get an external device or a smartphone app to create a GPS log or track file and sync with photos in post via timestamps.
If you don’t have a fancy camera that does GPS or Wi-Fi, and can’t recognize GPS add-ons directly, then this is probably your only bet. The good news is, you can choose whatever GPS receiver/app you want so long as you can get a track or log file from it. You may have to convert the track to a format your geotagging software understands (e.g., GPX), but the open-source GPSBabel can handle most formats. Your main concerns here are that you’ll have to sync the clock on the camera before each shoot, and you will want to discover if the GPS track requires timeshifting to match your camera clock (but again, software can cover most of this).
5. Adding location geotags manually via an application.
Many photo processing software packages (e.g., Lightroom, iPhoto, Picasa) allow you to manually add geotagging into the EXIF of a photo by locating where the image was taken on a map. Obviously, the main drawbacks here are the accuracy of the information, and that it needs to be done on an individual image/location basis.