If you have built one before why not do so again? Apart from the fact that you will enjoy it you can enjoy deciding just what spec you want, you won't have a problem with the supplier going bust or whatever, and if it goes wrong, you'll have 24/7 help on hand to put it right.
Having said all that, you probably won't save much, if anything. PC World sell a variety of systems at competitive prices, and you can take it home with you. Some of their makers produce computers that may not be too easy to upgrade later, with motherboards and other components that are specially made for them
If you are new here you may not have seen discussions about computers that come without a full copy of the Windows operating system, but have it instead on a hidden partition on the hard drive or as a "recovery" disk, which while restoring the operating system if it all goes pear shaped requires you then to go on and reinstall all your own files (if you have been clever enough to back them up) and software that you have installed yourself. There is an argument for buying from a manufacturer that will supply you with the proper disk (there are a few) or finding out how to extract and copy the system files from your hard drive. If you build your own, you will, of course, not have this problem