Which to get 2004 buick regal ls with 121000 miles or 2004 Hyundai sonata lx v6 with 130000 miles?

I'm asking for advice from ppl who have own these 2 cars. I needed a car thats going to last a long time and need to be on budget and please don't tell me go to Honda or Toyota or Nissan. I'm interested in one of these cars.

The one that has been driven more gently and has had better routine maintenance with records to back it up. If you don't have the knowledge and skills to do a proper pre-buy inspection and test drive then hire a reliable shop to do it for you. That will be money well spent if it prevents you from buying a p.o.s.

Its impossible to say with any degree of accuracy and without knowing the current mechanical condition.

You should take both to your mechanic and get the one he says is best.

I own a 2004 Sonata V6 for two years now, a friend has a 2003 Regal (same car as the 2004).
Both are quite similar in looks and performance, unless the Regal has a turbocharger, and in that case, I'd skip that car for too much extra maintainents, and unless you could verify that the Sonata has had the timing belt replaced in the last 30k miles or so, I'd skip that car for that reason. The Buick has a timing chain, not a belt. Also, if the Regal's intake manifold gasket hasn't been done in a while, that's around the corner too.
I bought the Sonata with 105k on it and knew it need some work done on it, which I do all the repairs at home.
The 2.7 Sonata engine & transmission, along with the Regal's 3.8 engine & transmission are both good & reliable. Even though the Buick has a larger engine, it will get somewhat better gas mileage than the Sonata.
Drive-train, steering & suspension parts are readily available and reasonably priced for both cars.
And at this age, both cars should be thoroughly inspected for steering & suspension wear & tear, including all the bushings and links, along with the brakes.
If you can't or don't want to do a good inspection, pay an independent mechanic to do so. The $100 or so spent can possibly save you many many times that in hidden, unknown repairs.
I drive my Sonata 65 miles to & from work daily, and it's as nice as anything else in the same class. See which one fits you best.

These cars are 14 years old. How they have been maintained during that time is going to be much more reflective than their make, model, age, and mileage.

Additionally, you can expect things to need replacing because many parts wear out based on age, not mileage.

Get the car that you want to purchase inspected to see what kind of work it needs.

Those are 14 year old cars with a lot of miles. There's no correct answer here.
It's a coin toss.