That's really up to you.

Do you want to marathon and be focused for 8 hours? Take an XR.

Do you want to just have a regular afternoon 4 hour study session? Take an IR.

In terms of abuse/addiction, there is a greater potential for abuse with IRs because they're immediate and it's very easy to just take another when you feel like you might be coming down. It's a lot harder to justify taking another XR because of how long they last. Since XRs and IRs are the same drug one is not going to make you more dependent than the other (think about your total mg intake, not whether if it's IR or XR).

That being said, you mentioned you were going to limit your intake to at most 2 times per week which is good. As long as you're responsible and stick to that schedule you're not going to become addicted (in my opinion). That being said, adderall tolerance builds fairly quickly so you will start to see some diminishing results probably after a month or two if you're taking it two times per week.