I would do all payments via PayPal. It's is by far the most popular. Whether a sale or purchase, when people plug their credit or debit (don't know if PP takes debit) cards into PP, I assume they are checked immediately as any vendor would get approval on a CC purchase.
Yes, many people don't know cashier's checks are still cleared manually, and often have to go thru many reserves. That's why scams using them are so successful.
You can go a full 3 weeks thinking you sold something successfully only to learn from your bank the CC was returned, and you are out of your money AND the merchandise you have already shipped.
Edit: for the record, CC checks CAN clear faster, but it is dependent upon various things. A CC issued by a major banks and with all transactions being performed in the same city, they can clear in a day or two. However, the smaller the bank issuing, and the greater the distance, the more time it can take.