So the conditions within a Campaign are always evaluated only for the contacts that are also in the Campaign.
New Campaign
Campaigncan only be selected as a contact source for Campaign. Some actions are not possible here.
So if you can wait a maximum of 10 min for the email to be sent, then I would take a Campaign. Then you can cleanly evaluate in the Campaign whether it was opened, clicked and downloaded. Here, one step is omitted (segment => Campaign), and the contacts are thus somewhat faster in the Campaign.
New individual form
Single forms are not tied to Campaignand support all actions directly in the form.
If the email is to go out immediately, then I would take a single form. Here you can send the e-mail immediately, then pack the contact into a segment and send the contact to max. Get 10-15 min into the Campaign. In the Campaign, you can then check, for example, whether the contact has already entered a phone number. But unfortunately it is not possible to reliably check whether the user has downloaded an asset.