Im torn between
Disneyland and Tinkerbell Half Marathon, opinions! Help!
Like
@Sleepless Knight said, there are a few things to consider...
1. Theme (affects the finisher medals, the participant shirts, and merchandise) - also Tink is more female-oriented, if that matters to you
2. Time of year - Mother's Day Weekend for Tink vs. Labor Day for Disneyland
3. Weather (May looks to usually be cooler but rainier than September)
4. Courses - If you google the course maps you'll see that they're similar in that they (obviously) go through the parks, but they do have differences (i.e. the Disneyland Half passes the Honda Center, along the Santa Ana trail and goes through Angels Stadium, where Tink does not). And not sure if you're planning on doing the half alone or the half in addition to other races, but I know that the 5K done during Tinker Bell weekend starts IN Disneyland park...which is pretty cool.
5. Coast-to-Coast --> If you're doing the Princess Half Marathon in the same calendar year, you can get the special pink Coast-to-Coast medal ONLY if you do the Tinker Bell Half. Any other WDW/Disneyland race combination in the same calendar year will get you the blue C2C medal.
Your best bet may be to Google race reports from people that have done the races before...that may give you an idea of what to expect.

But in the end you're still running at Disneyland, so I don't think you can go wrong!