An evening flight from ASE? The non-stop ASE-SFO is probably the most likely to go on time. That said, flying out of that airport is always a gamble.

I would consider this connection too close for two separate tickets unless you can check your bags through at ASE and get boarding passes for the BA flight at ASE. Since these are likely two separate tickets, UA and BA may just leave you stranded if there is any problem with the ASE-SFO segment.