Need help with identifying correct Maxxis tires

[RottieMom]

I'm going to go ahead and replace my trailer tires sometime in the next month and I decided (based on a lot of feedback on this forum) to go with Maxxis tires. What I'm unsure about is which ones to get-- 205/75R15 come in 6ply or 8ply and then there's 22575R15 that comes in 8ply. I don't understand the difference between the 205 and 225. Can someone provide some guidance? Thx.