I haven't gotten a single Hero and Monster collection, yet. Since RtL is the main way my group plays Descent is there any collection that is better than the rest or are they mostly the same?

Well, it depends on what you like.

For example, Bonds of the Wild adds new familiar based heroes. Two scouts and two mages.

Just do some research on what types of monsters you might want to face more.

Otherwise, just buy the cheapest one you can find online.

Each pack has it's own flavor to offer, so I'd suggest looking at what you're looking to add to your collection specifically. Never have enough warriors to choose from? Go shards of everdark. Feel there aren't enough female heroes? Pick up Crusade of the Forgotten. Familiars are your thing? (as said) bonds of the wild is for you. You might also pick based off the monsters.

I haven't gotten a single Hero and Monster collection, yet. Since RtL is the main way my group plays Descent is there any collection that is better than the rest or are they mostly the same?

Take a look at the descent wiki: http://descent2e.wikia.com/wiki/Descent:_Journeys_in_the_Dark_(Second_Edition)_Wiki

Not all heroes are created equal, so look through and find who you want to play and see what monsters are cool.