If you build your PBN site, then it's a waste of resources IMO to use it as a tier 2 and point it to a web 2.0 buffer. Both expired web 2.0s and expired domains should be pointed to a money site. Of course, there are some other techniques, but for the time being, let's just focus on the basics.
Difference between the web 2.0s and domains is day and night. If your keyword is at least somewhat competitive, you won't rank top 3 with web 2.0s alone. You also aren't in control of your web 2.0 properties and they can be deleted at any time. They are however safer than PBN links. Another advantage is, that they are cheap.
PBN domains are significantly stronger, there is a risk involved with them of course if you don't build them right (and even if you do to some extent). They are more expensive, considering the cost of content, hosting, registration etc, but you own them and they won't get deleted just for lulz.
If you are planning to rank some medium competition keywords, then it will be very very hard to rank them without PBN links or without guest posts or other strong and more expensive links.