Because the story of the expansion happens before the events of the main game.

TOB makes somewhat better use of the engine. TNO gives the impression it used id Tech 5 because it would be unthinkable not to use id's then-latest engine for a Wolf game. I haven't played Rage (or ET:QW on id Tech 4) but my impression is that the Megatexture stuff was intended to showcase big, open environments with nontiling textures. TNO applies it to a setting with enough brutalist architecture for the Nazis' use of concrete to be a plot point. So you get all the downsides of the engine - texture pop-in when you turn around, even on fast hardware; poor texture resolution at close range - and you're often surrounded by concrete and bricks and so forth that could have been done better with traditional tiling textures + decals. There are some nice interiors where the artists/mappers have piled on the details - especially the Kreisau Circle headquarters, visited multiple times - and the engine's a nice fit for ruined areas, but only the destroyed railway bridge really seemed to be making fullest use of the tech. The moon base especially felt like an id Tech 4 game: a techbase built with fancy wall panels and a very short outdoor area.

By the time of TOB the devs had got wise to this, so besides making a bit more use of open scenery (they make sure they get full mileage out of that Alpine valley with the cable cars) they piled on polygonal detail in some of the narrower areas: castle walls with each stone individually modelled, etc.

Gameplay-wise, TNO makes a better first impression, whereas TOB opens with forced stealth and few tactical options. They both have a 'perk' system (unlock improvements for your character by killing enemies in certain ways) but it's easier to plan around in TOB because it usually lets you keep your equipment between maps, whereas TNO keeps taking it away.

Neither of them can quite decide whether it wants to be an FPS with explorable maps or a linear shooter: you're encouraged to explore to find hidden stuff, especially in TNO, but then going the wrong way can see you passing a point of no return, usually followed by a checkpoint save in the one save slot with autosaves only. They possibly think this is replay value. TOB is better about this basically because there's less to miss.

TNO wants to do everything, so through sheer variety and experimentation it has more highs and more lows (scouring a nonhostile map for someone's lost belongings is an unexpected sidequest in a Wolf game). TOB has more focus on being an FPS.

Uff, tough choice. I also played both this year, and enjoyed them both. I would say I liked The Old Blood little better.

Thanks for the feedback. Still mulling over these two (and other games haha)

I would definitely recommend both games! They both have a fantastic dieselpunk atmosphere! The Old Blood is like a separate campaign but with more action. The New Order has the bulk of the main story.

Now if only GOG could get Wolfenstein 2009 as well...

Do you guys advice to play them chronologically or by release date?

Doesn't really matter as it doesn't affect the fun you'll have when you shoot nazis in the head.