the .300 will be a better option. It is always better to have a stiffer arrow than a weak arrow. to reach 500gr you will likely need to add weight at the insert with most arrows. there are a ton of arrow options that feature an outsert, Easton HIT insert system, or the goldtip FACT weight system, that can be used to tune the tip weight of your arrows.
A ton of people have great luck with the FMJ arrows and like them because they are heavy. I like a heavy arrow, but I think it matters where the weight is in the arrow. I prefer an arrow with higher tip weight and lighter weight over the shaft. This increases a measurement known as FOC or front of center, which refers essentially to the location of the balance point of an arrow.
Think of it like paper airplanes when we were kids, to make them fly straighter and farther you would add paper clips to the nose right? the same is true of carbon arrows in my opinion.
The limiting factor here is the spine of the arrow. The more tip weight you add, the weaker the spine of the arrow. Depending on the IBO speed of your bow this could be problematic.
If I was you, I would first go to a shop and find out how short you can cut your arrows and shoot them comfortably. I prefer the end of my arrow to be past the shelf so that broadheads don't cut my fingers at full draw/release. Shooting the shortest arrow you can will help stiffen the arrow and allow you to add more tip weight.
Although I don't shoot them any more, the goldtip FACT weight system allows you to add and remove weights from arrows and experiment to find out what weight combination is right for you. It wouldn't hurt to order a half dozen and do some experimenting. once you figure out what spine/tip weight combination is right for your bow, you can branch out to other brands and find similar setups.
Good luck
