Nightfall vs Martial Law

Both being stand alone which would be the better buy?

Also, how do the factions play in these games? I understand it's werewolves vs vampires vs humans, but does that come across any at all? I mean do I have a deck of humans and face an opponent's werewolves, or do we all end up with random cards in our hands and the factions really don't matter thematically?

I like how in Thunderstone everyone is building a party and equipping them to go face down monsters in the dungeon. I get it and feel it. I'm worried that the Nightfall series is just players attacking each other with vamps one round and humans the next, having all these factions in our hands, and seemingly little tie to the theme.

I'd get Martial Law just because it includes additions that make the game better and Nightfall doesn't have them...and since they're the same price, no reason not to just go straight to the top. If you like it, then go for Nightfall just to increase the card base.

As for the theme, there's better (ugh) synergy between like factions but yeah, you can have a deck with all three. It's not really a big deal. If you want, you can fix the card sets so that you're only playing with certain ones and then draft just the factions you want. That's totally viable.

There's actually FOUR factions, including the ghouls. Some cards in Martial Law are pushing more towards having a theme to your deck, with cards that specifically target vampires or werewolves.

I personally hope that as the expansions keep coming, eventually a truly faction-based game mode evolves. If I was only buying one, I'd get Martial Law, but if you like it you will definitely want to get Nightfall. I use the sets mixed, but use the wound cards from Martial Law since they include a new wound effect.