This question is very typical, and the decision that needs to be taken is fundamental.
Following you find some hints that hopefully help to take this decision:
A) In case you deal with prospect data that contains only a few attributes, for example the e-mail address only, it is often not allowed to create such prospects as business partners in the CRM system. The CRM system itself can store such data, and it is also possible to configure the data exchange via CRM Middleware in a way that such prospects are not replicated to a connected SAP ERP. But usually organizational guidelines exist that need to be followed for creating a business partner in the CRM system. With an e-mail address only it is often not allowed to create a business partner.
In this case I would recommend to create such prospect data as marketing prospects in SAP CRM.
B) Besides it can make sense to explicitely separate the data of prospects where you don't know at all yet if you can start a real business relation with them; if they are interested at all. They perhaps only registered on the company's web site or talked about the company's products on Facebook or Twitter. But this doesn't mean that they have a quite serious interest in the products, or that they would even buy some of them at some point of time. In many countries the data privacy regulations are quite strict: It is not allowed to store data from persons, with which no business relation exist.
In this case I would recommend to create such prospect data as marketing prospects in SAP CRM, and convert them to business partners as soon as a real business relation starts.
C) In case the company deals with prospects in a very close and extensive way from beginning on, somehow not dealing with prospects that are not considered as being quite serious about their company, creating them as business partners in the CRM system can make more sense. Only business partners can be used in activities, leads, and opportunities, to mention some examples for business transactions that are based on the one-order framework of SAP CRM. So if such business transactions need to be created in SAP CRM for the prospects from beginning on you have to create such prospects as business partners in SAP CRM.
Some final remarks:
If the decision is taken to use marketing prospects for storing prospect data in SAP CRM, be aware of their short life-cycle: Either convert marketing prospects as soon as possible to business partners, or delete them if you can't establish a real business relation with them.
The purpose of marketing prospects is to support you in a very defined and controlled phase of time in your marketing activities.
I would recommend to define exactly what needs to be reached by marketing, how this can be reached, and in which time frame this needs to be reached.
This helps you to always have an overview of the prospects, and the company's success with these prospects.
