I used to play wow about 8 years ago. I tried to get back into wow when legion was coming out. Played my alliance Hunter from mid 60 to 78 and was completely bored. I recently tried the free version. I found Warlock to be ok but not great, too similar to my memory of the hunter. Monk seemed interesting. Discovered that I was having the most fun with a Paladin. This is my first time playing on the horde side so I'm going to try leveling this one without a boost. I would like to see the alliance stuff but don't know if I should use my boost on a death knight or save it for a character that starts at level 1 a monk or a mage. Is the content different enough in the expansions that I would enjoy the story if I leveled death knight on the alliance side after doing the content on my paladin?

Ask yourself if you would want to fill in different roles or play one exclusively (tank/heal/ranged dps/melee dps). Dependent on that narrow down the possible/optimal classes.

You can level pretty fast from 1-60 nowadays, so in the big picture you don't gain much out of boosting another class but death knight if you lean towards that one.

In the upcoming expansion BfA the alliance and horde leveling experience will be more noticeable different, so it would make sense for you to use the boost for a character on the opposing faction.