Perennial vs Annual
After fifteen years of buying both in my zone 7b clay, here is the honest call on when a perennial earns its keep and when an annual is the smarter spend.
A perennial lives three or more years, dying back and returning each season; an annual completes its whole life in a single year and must be replanted. Perennials cost more up front and bloom for a shorter window, but they pay you back for a decade. Annuals are cheaper per plant, bloom hard from spring to frost, and let you change the picture every year. In my zone 7b beds I use perennials as the bones and annuals to fill the gaps and the pots. Most real gardens want both.
This is the first decision every new gardener trips over at the nursery, and the labels do not help. I garden on heavy clay in zone 7b, mostly full sun, and I have killed enough of both to have opinions I trust. The short version: a perennial is a long-term resident, an annual is a seasonal guest, and the trade is durability against display.
There is a third category worth naming so you do not feel lied to: biennials, like foxglove, grow leaves the first year and flower the second. And plenty of so-called annuals are really tender perennials we treat as annuals because they cannot survive our winters. For most planting decisions, though, the perennial-versus-annual split is the one that matters, so that is what I will settle here.