It is one of the biggest decisions that a bride and groom will have to make. A lot of couples can afford both, but some can not and therefore have to decide which to have.
There are many factors that effect this decision. Here are the most common ones:
1, Activities. The activities within the wedding day, make a big effect when deciding which of these art forms is the most important. If for example you have fireworks or a band playing at your wedding, you are going to be much more likely to hire a wedding videographer, because a videographer records sound and movement and both of these activities have these in abundance.
2, Effect of seeing other peoples weddings makes a huge effect also. If you were at a wedding and the videographer was getting on every ones nerves by putting the camera in people's faces, then you are much less likely to have a videographer at your wedding because you do not want that sort of thing going on.
Also if you have seen a friend's wedding video and it was no good, you are not likely to want one yourself. If how ever you have seen your friends and it was really good, then you are much more likely to hire one yourself.
3, Which art form the bride and groom like the most. Some people are complete film buffs. If they are, then they are much more likely to want a wedding video.
4, Permissions are also important when deciding. If the vicar will allow a videographer into the church during the ceremony but not a photographer, then that will push the bride and groom towards a wedding video.
5, Can the bride and groom get one for free. It is often the case that the bride and groom will know someone who can do one or the other. If they have a friend that is pretty good at photography, they may decide to get him to do the photographs so that they can pay for the wedding video.
In summary there are a lot of factors that effect which to pay for, but in my experience these are the most common. It is a massive decision, which will effect you for the rest of your life, so it is worth taking the time to think it over.