Garmin vivoactive 5 VS forerunner 255

Im doubting between getting a vivoactive 5 or a forerunner 255. Can get the forerunner for 250e and the vivo5 for 299. Im a starter in running. Goal is a 10k and once in my life a half Marathon. 

I like the amoled of the vivo5. But the 255 had more setting to run using heart rate (I’m training on heartrate). But it’s strange that the 255 is (next to this temp offer) everywhere way more expensive than the vivo5. Is it the bigger screen? What I’m a missing?