Get one with a recognizable enough brand, since diagnostic stuff these days (and for several years now) will need a firmware update or two over their useful life, while the generic stuff or unknown brands who merely put their brand on the generic stuff are much less likely have ongoing development or support.
Personally I haven't used one in years and years. The affordable ones (e.g. <$30) are usually going to be next-to-useless and I have never had much need for the good stuff (e.g. >$70) with real professional test features or capabilities such as onboard CPU that can bring-up a system from the POST card. Maybe a couple times in 20 years, I would have liked to had access to one of those full-featured ($$$) test cards but not much more.