Definitely go with Interfolio. If you submit directly to AACOMAS, you cannot have access to those letters again and they are essentially locked in. Through Interfolio, you can save those letters forever and use them whenever you need to. It's also much easier for letter writers to upload to Interfolio because they literally click a button to upload it, whereas with AACOMAS they have to make an account with a password and a bunch of nonsense. Also, if for whatever reason one of your schools doesn't have one of your letters, you can easily send it directly to that school through Interfolio. You can't do that through AACOMAS, they're locked in. It's only $4 to send a letter anywhere. You can send each letter to AACOMAS so your schools still receive them with your application, but then you still have them in your Interfolio account for future use.
Also, make sure every letter is on official letterhead and PHYSICALLY signed by the letter writer. I found out the hard way how strict of a requirement that is for many schools.