flatbed+adapter versus film scanner
A flatbed can cost from under $100 and the results are surprisingly good (at least with a decent brand of scanner) but the process is vastly time consuming. I've found flatbeds to be a bit flaky -- sometimes having problems with white slide mounts affecting colour/exposure -- and sometimes not finding the slide at all.
Dedicated film scanners cost from $400 up to thousands and give consistent results (ignore products for dramatically less as they are basically digicam sensors with a film holder).
The worst problem I've experience with mounted slides (as opposed to neg film in storage pouches) is older ones suffer surface coating from air pollution which (at best) disperses the scanner's light and creates odd exposures. This means that older slides and badly stored strip film may need washing and careful dust free drying. A nightmare.