The reason no one replies to this post just because you are asking a down to earth question :0). Don't worry, I have enough sympathy to cordially reply to you. Come to this web site and you will become a valve expert
Actually ball valves aren't great for throttling, unless you're talking about something like Fisher's vee-ball line. With that exception in mind, usually you'll only want to use them for block valves, like gates & butterflies. The advantage they have over gates (depending on size) is the quarter-turn operation makes them easy to fit with relatively low torque actuators. For throttling you'll usually want to use a globe valve. For instance, in the natural gas field, emergency shutdown valves are usually quarter-turn ball valves rigged with pneumatic piston actuators. Manual block valves at battery limits and such are usually gate valves.
I'm sure by now this is a dead subject, but I'm dying to add my 2 cents! As most have pointed out that these valve are normally used as block valves. the advantage to the ball valve is that it's a 1/4 turn operation, and after having to deal with an old rusty gate valve you'll quickly learn why the 1/4 turn valve are favored. UNLESS you have to pay for them. Ball valves start to be come more expensive in about the 3" size and higher. That's when gate valves start to look more attractive. Be that as it may, dimensionally gates and ball valves are the same size to about 6", so you can change you mind later down the road. AGAIN, my 2 cents! ...Mark