Band
I love live music and a band can really get the crowd going. Live guitars and drums are awesome! It's cool to watch a band do their thing (slightly cooler than watching a DJ press buttons). They put their own spin on songs which can either be a good thing or a bad thing (some people love new interpretations and some people love the originals).

Bands are a good choice if you really want to impress your guests and you have a large budget - they are going to be the most expensive option as there will be 3 or 4 people in the band. They only will know a limited number of songs in the pop/rock genres and so probably won't be able to take random requests.  

DJ
DJs can play literally any type of music all night. They are experienced in reading crowds and should know what song to play next to keep the vibe going. They make sure songs flow into each other smoothly and might even do some fun mashups.

When the dancefloor is open they are usually only playing the best parts of each song, meaning you get to experience a lot of different songs and moments rather than listening to each song in full.
The best example I can give is the song 'Ice Ice Baby' by Vanilla Ice. The first verse goes off, but hardly anyone knows the second verse - a good DJ is going to be cleanly mixing out of this song as soon as the first verse finishes to keep the dancefloor energy up! 

A DJ will also have been to plenty of weddings, so can help assist your MC and wedding coordinator to make sure the day runs smoothly. 

DIY Option
Asking a friend to help with the music can work, but it is a risky option.
It will obviously be the cheapest, but if something goes wrong on the night then your friend might not have a backup plan and they probably won't be experienced enough in reading crowds to know what songs to play at what time. They also aren't going to be able to relax and enjoy your wedding!

The main thing to remember if you choose the DIY option is to make sure you hire some good speakers - if the music is not loud enough it is a real vibe killer. The internal speakers at wedding venues won't be anywhere near loud enough for a dancefloor - you need to make sure you bring along your own proper speakers. 🤓