I don't think comparing 1950 feudal Tibet to modern day Tibet under Chinese rule is entirely fair. The assumption being that an independent Tibet wouldn't have changed at all in the last 70 years.
Also, "figures show that since the early 1960s, the Tibetan population has been increasing, probably for the first time for centuries" (https://en.wikipedia.org/wiki/Human_rights_in_Tibet).
There are still human rights issues in Bhutan, but it's a far cry from where they were 70 years ago.
According to [1] Bhutan's HDI was 6.17 in 2018, while according to [2] the regional HDI of Tibet was 5.85. So at the very least they're not particularly far apart from each other.
[1] http://hdr.undp.org/en/countries/profiles/BTN
[2] https://en.wikipedia.org/wiki/List_of_administrative_divisio...