What are the advantages/disadvantages between using 15# felt versus closed cell foam as an isolation strip under stud track?
Isolation against what? I assume we are talking about non-load bearing walls.
Any floor will deflect enough to totally compress 15# felt. Foam may not be much better and will transmit more load than you assume.
Back in the day when I worked in Calgary, Alberta Canada, the Alberta equivalent to the Northwest Wall and Ceiling Bureau recommended setting the bottom runner for non-structural metal framing on two rows of double sided foam adhesive tape to, 1)secure the runner without mechical fasteners, 2) fill in bird baths between the runner and track, 3) mitigate sound flanking at floor and vertial walls.
I have personally done this and it was a bitch to remove the runner and what is left of the tape. Foam tape was also recommended between vertical studs and concrete or CMU for some of the same reasons noted above.
I have not found this to be standard practice in the PNW or elsewhere in the US.
We use foam under cold formed studs used as masonry back up (or other exterior material) as a way to block air infiltration between the uneven slab and the stud track. I don’t think #15 felt would fill the space.