Assuming you mean RG58 vs RG58U, the answer is that RG58 has a solid center conductor, and RG58U has a stranded center conductor. 58 for permanent installs where the cable won't be subjected to repeated flexing, and 58U for installs that need more flexibility, tighter bend radius, etc. such as mobiles or temporary connections.
But yeah, RG58 is generally crap for anything other than very short runs, and the attenuation gets worse with higher frequencies. Avoid it if possible. Note that if you do have 58U and need to attached a connector to it such as BNC, make sure the connector (center pin specifically) is compatible with stranded center conductor.