Salesmen! :

They rarely know anything about the things they sell.
The first salesman was a simple waste of human flesh.
The second salesman was talking (politely phrased) out of his posterior.
It's more likely that the shop you went to has only one BNC connector style in the proper impedence. Most pedestrian 75-ohm cable is in the aluminum-shielded variety, for which the crimp-style connector will be needed because I still don't know of a realistic procedure for soldering aluminum shields to anything.
75-ohm cable
can be had in copper braid shielding styles, but it's not as common, and the center conductor is usually stranded. It is for this style of cable that the solderable connectors are intended.
If the cable you have is the run-of-the-mill aluminum shield type, then you're stuck using the crimp-installed BNCs.
Please suggest to both salesmen who gave you "advice" that, they would be better-off considering a career in used car sales. At least the customers
expect to be lied to, in that type of setting.
Yeah....it may be best to let it go for right now.