The biggest difference is that an SPA is the sale of all shares, and an APA is the sale of selected assets. Therefore, they are both different transactions and have different procedures.
What should be included in a buy-sell agreement? Any stakeholders, including partners or owners, and their current stake in the business' equity. Events that would trigger a buyout, such as death, disability, divorce, retirement, or bankruptcy. A recent business valuation.
Choice – on an asset deal buyers can pick and choose what assets and liabilities they want to take on, leaving what they don't want behind. That way risks can be minimized. On a share deal there is additional risk because the buyer gets everything “warts and all”.