Location is obviously a huge factor in choosing a race. If you are lucky enough to live near a race, then that may be your clear go-to. Alternatively, you can choose them in terms of where you want to vacation, as I do.
But if you are a back-of-the-pack racer, or worried about finishing, then it would also be good to consider the course difficulty as well. Ironman has a search feature so that you could look for a flat race. The only two courses I have done so far have been in this category, and I am glad that I started there – especially since I have not had that much leeway in terms of time.
Of course other things can make a race far more difficult than a few hills such as weather which is difficult to control. For example, I have done Western Australia twice. The date hadn’t changed, but the temperature was about 20 degrees different each time (Fahrenheit).
Elevation is also a good consideration for races such as Boulder, Colorado. I cannot imagine doing that one now that I live at sea level.
If you know a group who is going to a race, that is probably the best way to choose if you would have a group to train with and enjoy that sort of thing.
Just a few thoughts. Most importantly, don’t overthink which course if that will paralyze you into not signing up at all!!! Just do it!