From my viewpoint... (in addition to everything everybody else has already said...)
The Acura: https://www.kbb.com/acura/tlx/2015/
To me, the biggest CON here is that the 2015 TLX is a "first year" model (or the first year of production). OTOH ... Acura is the luxury brand of Honda.... which are generally extremely reliable cars.
The Nissan: https://www.kbb.com/nissan/sentra/2023/
Having owned more than my fair share of Nissan products over the years, I can honestly say that it is one brand I would never consider again. Why??..... all of the ones I have owned seem to start needing serious repairs/work the 4-5 year point.
Another point: If a 2015 and a 2023 are in the same price range.... what does that tell you? Well, it should be telling you that the 2015 is a "higher end" (read quality) and the 2023 is at the lower end of the "value" spectrum. And the 18k milage means the car averages 2K miles a year so the owner is retired or works from home. Generally speaking older folks tend to be a lot more gentle on cars than you hotrodders.... 😆
Additional things to consider:
-Difference in insurance cost
-Repair costs difference
-Maintenance records
-CARFAX reports - PLEASE GET THEM FOR EACH VEHICLE, no matter the cost. Carfax reports can tell you a lot about the vehicle and its history.... which can save you a boatload of money in repairs/fixes, especially if either car has been in a serious accident or flood.
-Tires... check the date codes on the sidewalls (that you know how, will impress the person you are buying the vehicle from... and is a great bargaining point....LOL). Tires have a "life" and that life lasts 6 years. On the sidewall of any tire, there is an oval somewhere, and in the small oval are four numbers IE 14/15. That particular code means the tires were manufactured in the 14th week of 2015. In this example, THOSE TIRES ARE EXPIRED AND NEED REPLACED! Check closely on the Acura.... because the Nissan is a newer vehicle and should have tires from 2022/2023...
-Run the VIN numbers and check for recalls
-Look closely at the glass on each vehicle for chips, cracks and so forth.
-Speaking of glass.... look closely for any signs of "dry rot" (cracking) on the seals (rubber on the edges of all glass).
-Look at the wipers.... are they flexible and in good condition
-Look at the front end lights.... does the light/cover show signs of age (cloudiness?). Are there any cracks or damage?
-Look under the hood. Is it clean? Does the battery show signs of corrosion on/near the posts? What is the date code on the battery? (Where I live we just "rent" glass and batteries.... so if the battery is over 4 years old....I just replace them cuz I HATE to get stuck in the middle of nowhere with a dead battery... I refer to that as the MURPHY factor... 😆)
Congratulations....