If a Lead is deleted in SFDC, then the Lead usually stays in Marketo and is marked as "Deleted in SFDC". This lead can be resynced to SFDC, but it won't be "restored" if that happens.
If the Account is Deleted, then i don't know. I doubt the Account can be deleted with existing Contacts. Contacts themselves would act the same way if Deleted in SFDC, but they would return as SFDC Leads if Marketo resynced.
You can delete an account in SFDC without deleting the contacts if contacts without accounts are allowed in the SFDC instance, which usually not the case.
But I have seen it possible to sync with Marketo even if the account was no longer sync'ed, which means that I suppose that deleting the account (provided it's allowed in SFDC) would not immediately affect the contact... but would result in issues on the long term.