First of all there as many opinions as runners about how to choose running event. I will cover some good tips and points which are good to consider either running on marathon or half marathon.
Profile and terrain
If you wish to new PB or fast marathon. Check the route, often marathon organizers’ show profile in their website, the flatter the better, as the every hill reduce the speed. Also prefer more city run where the ground is asphalt as they are often more flat and not soft which makes running slower. Of course if you more adventurous and want challenge trail events and hilly events are for you.
Normally marathons start is early in the morning, as the weather often get hot during the day, so most of the runners have time to finish before there are too hot. And also as organizers has to reserve enough time to the slower runners has to time to finish the race before the night. There are some events where the start is middle of the day, so take an account weather and season, if it hot summer, the run might come way too hot.
Major marathon or small event
There are pros and cons both of event types. Major and bigger marathon are often good as the organization or big so there are very good services and a lot of audience to cheering during the route. Also a lot of other runner, so you don’t need to run alone, but also this might slow your pace down, especially at the beginning and in the water stations. Smaller events are often good for the getting new PB, as there are not massive crowd, also there are not ballot system or anything else, so the registering to event is easier.