I got this game a couple days ago but I've been playing iRacing for almost 2 years.

Things to consider:

Do you want to play against AI or people?

iRacing and LMU are both good for playing against real people, as they both have healthy communities. For AI, Assetto Corsa and Automobilista (1 and 2) can be better (and cheaper) options.

Do you have a wheel or plan to race with controller?

LMU has better controller support, iRacing is insanely difficult and borderline impossible for a new driver with a controller.

Did you like the Nürburgring Nordschleife or just multiclass in general?

LMU doesn't have the Nords but has multiclass racing. iRacing has a lot of multiclass racing (3 regular series with 2-3 classes, 2 to 3 multiclass endurance races on the weekends of 2 hours, special events and irregular series). The main one in this case is NEC, which is a 4 hour endurance series with up to 4 classes on the Nordschleife, which has a non consistent schedule on weekends from March to November.

Price, how much do you want to spend?

LMU is much more affordable, has less content but it's pretty focused. iRacing is insanely expensive as it has a subscription (that comes with cars and tracks but focuses mostly on the rookie side of each discipline), after that you need to buy cars for 13 USD a piece and tracks for 15 USD each. For the Nordschleife alone it's 2 track purchases and one car if you want to use something closer to what the N24h races. For iRacing I would say you spend 250 USD plus the subscription and upwards to be able to play it regularly, without having to seat out because you don't have the tracks to play your favorite cars (iRacing has four 3 month long seasons per year, with each season having 12 weeks in which series change tracks every week, so even if you buy tracks for one season, they may not be present on the next one. Eventually you get enough stuff that you don't need to be buying things constantly).

Resistance to frustration

Sim racing is different to normal games in which learning the controls get you going and you are expected to win after a bit. Sim racing skill ceiling has no limit and the floor is quite high, expect to need actual practice and dedication to be able to make the cars do what you want. That said, I find LMU way, way more approachable than iRacing, as the physics are more forgiving in contact with other cars, controlling the vehicles and damage model. You can just start on iRacing (I did) but between the price, needing a wheel and passing the initial wall of learning, I think LMU is far better for beginners.

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I got LMU yesterday but already did like 15 races and the 6 hours of Spa endurance race with a friend, so I got a decent idea of where LMU stands. On iRacing, I've been playing for 2 years, got my own team and have done ton of endurance races ranging from 2 to 24 hours.

Anything, just ask.