Circumstance, Budget, and Preferences determine the answer to the question.
For best results, and assuming a Stereo system, if you don't have Electronic Bass Management for BOTH the Front and the Sub, the usefulness of a Sub is limited.
Further Subs are not cheap, and the need for ultra-deep bass in music, with a few rare exceptions, is not really necessary. If you are down close to 30hz, then you are close enough.
If you are buying all new equipment that is very different than upgrading an existing system. If you have bookshelf, and your priority is to have deeper stronger bass, it make sense to add a Sub. Assuming you have the common sense to set it up properly.
If I were buying a new system, I would rather put the considerable cost of a Subwoofer into better Front speakeers. Beside, I could always add a Sub next year.
Of course the answer depends on whether the amp is a AV Receiver or a Stereo Amp. For Surround Sound, most would say that a Subwoofer is a must, or at least preferred. However, in a pure music Stereo system, most would likely say get good floorstanding speakers.
There are some here who swear by a Subwoofer in a pure Stereo Music system. That aren't wrong, but I think they are wrong in why they think a Sub provides an advantage. They would say the advantage is deeper bass, I would say it is simply more bass but not necessarily deeper bass. With a Sub, you have a huge speaker pushing massive amounts of air and you can control its volume independently. For me, that's a disadvantage, but for those who prefer this method, the Sub has a distinct advantage.
So, you haven't really given us enough context to answer the question. All the other not-listed parameters are what determine the right course of action.
It is not as simply as this speaker combination vs that speaker combination, the determining factors are really in all the other not mentioned parameters. What size is the room? Stereo or AVR? What equipment do you currently have? What is the source of your content? Movies or Music, which is the highest priority? What is it specifically that you want from the system? And so on....?
Steve/bluewizard