Having done federal time myself, the honest answer is that federal is generally better, but the comparison is more nuanced than most people realize.
First, on your question about the population being international, that is not accurate as a general rule. Federal prisons house American citizens convicted of federal crimes alongside some non-citizens, but the population is not predominantly foreign nationals. Federal crimes include non-violent drug trafficking, white collar offenses, RICO charges, human trafficking, child endangerment, terrorism, and any criminal activity that crosses state lines or involves federal jurisdiction. The US Attorney's office prosecutes these cases rather than a county district attorney, but the inmates are largely American.
State prison populations tend to be more mixed in terms of offense severity because state charges range from simple theft all the way up to rape and murder. Classification and housing assignments are based on the crime, sentence length, and criminal history, but the range of people you are housed alongside is wider.
On conditions, federal wins in most categories. The Bureau of Prisons operates on a budget of about $7 billion annually, which shows in the food quality, commissary selection, facility conditions, visitation setup, and the caliber of staff. Federal corrections officers are generally better paid and better trained than their state counterparts. Some state systems, Florida and Texas being notable examples, do not even have air conditioning in their facilities. Federal prisons do.
The one area where state has a clear advantage is phone time. Federal inmates are limited to 300 minutes per month, which goes fast. State systems are often more generous with phone access, which matters enormously for staying connected with family.
Security level determines everything in the federal system. A federal satellite camp is among the easiest time available anywhere in American corrections. A federal maximum security penitentiary is among the hardest. Where someone lands on that spectrum shapes their entire experience far more than the federal versus state distinction alone.