Normally generics are cheaper. You'd have to check with your pharmacy. Also not all insurance policies cover both, so that's another thing to check. In the end, which one you take should be up to you.

Here's what Wikipedia says about the two:
"Biosimilars
Recombinant teriparatide is sold by Eli Lilly and Company under the brand names Forteo and Forsteo. In June 2020, Alvogen, Inc, Pfenex Inc.'s commercialization partner, launched teriparatide injection (Bonsity) in the United States. Teriparatide injection was developed by Pfenex Inc and approved by the US Food and Drug Administration (FDA) in October 2019.[1] Teriparatide injection is pharmaceutically equivalent to Forteo (that is, has the same active ingredient in the same strength, dosage form and route of administration) and has been shown to have comparable bioavailability. These characteristics allowed the product to be approved under a 505(b)(2) NDA for which Forteo was the reference drug. It may provide a lower-cost teriparatide option for increasing bone density in patients at high risk for fracture, and is FDA-approved for the same indications as Forteo, which means it can be used for the same patients as Forteo, including new patients and those currently responding to treatment.[25] "

You can access the full Wikipedia page here: https://en.wikipedia.org/wiki/Teriparatide