For most people it is an easy decision depending on the state or time of year it is. But there are factors to consider when making a decision.
Benefits of an inside ceremony:
There will be no worry about the rain or snow or temperature. There can be more options about decorating inside. And the inside ceremony can be done anytime during the year.
Negatives of an inside ceremony:
It maybe too small if couple has a lot of guests.
Benefits of the outside ceremony:
There usually more room since there are not four walls holding everyone in. The photo shoot has more natural lighting for better pictures. There is less to decorate so it would not take as long to decorate.
Negatives of the outside ceremony:
Bugs and the outside elements during the ceremony will not be fun. Some outside venues are blank slates so couple may need to rent furniture for the ceremony. Power maybe an issue since it is outside.
Before the decision is made, it is good to check if the venue has a back up plan in case being outside is not favorable. Also its good to check on the power source is close by and functional for the outside event so the speaker(s) will work for the ceremony.
Good communication is key with your venue connection in case there is a sudden change and the outside ceremony has to go inside. Hopefully they can contact the couple a few hours before in case of the sudden change. Djs can set up a ceremony event in 20to 40 minutes so they have to be in the loop in case of a change.