UiPath Studio is the designer but when you install it the Robot Service is already installed. So from a development perspective there is no difference. However, when you go into production you need to purchase licenses so if you are running Back Office robots you need to purchase back Office license to run these.
If, however, you still require the use of Studio (typically most people will) then you can continue to work on those particular machines with just the Studio license which is a little cheaper.
At present you cannot install the robot service separately, so you have to install Studio on all machines you wish to run UiPath on regardless.