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.
The buyer and the seller agree that the LOI is not a legally binding contract, except for certain clauses such as confidentiality, exclusivity, and governing law. The LOI is subject to the execution of a definitive agreement that contains the full and final terms and conditions of the deal.
A letter of intent can be either be a non-binding or partially binding document which is initially used between the parties when the agreement has started and before any of the formal work of the agreement has commenced and been finalised, it shows the parties wish/desire/interest to proceed further with this ...