You mean sharing entities, right?
The answer is ii), all data will be shared.
There is an option to create mutli-tenant eSpaces that behave like i) but they do not support Run/Debug and usually become harder to maintain, so if you need that I would suggest that you rather implement that behaviour programmatically with an additional attribute.
Regarding the sharing of web screens in version 4.2 it is still not possible to do it directly.
For now you can use a Dynamic URL in an External Site (make sure you use the EncodeUrl() function in parameter values).
Just a small comment regarding multi-tenancy in Entities: you don't need to have the eSpace as Multi-Tenant to have the entities behave like i), you simply need to mark the entity itself as being multi-tenant.
If you leave the entity with the default behavior, then you get ii). If you mark it as being multi-tenant, then you get i).
Check these links for more details:
This can be useful e.g. in a common entity logging activity, where it is interesting to filter by activity in a given eSpace.