I played Smash 4 for a couple of years. It was fun until I started playing against Cloud and Bayonetta and other characters more and more. My characters I did play were significantly weaker than these other characters, some of whom were brand new and given lots of attention and love, whereas my characters were not so blessed. It was frustrating to want to play on equal grounds against these characters and being unable to. So when I heard about and was introduced to Project M, I very quickly realized the significant flaws of Smash 4 that had already been beginning to dawn on me. After playing it for a short while, I had lots of fun with the game, much more than I had playing Smash 4, so I quit Smash 4 and haven't enjoyed playing it or watching it since.
I had invested about $50 into Smash 4 and now I want nothing to do with it. ~$50 into Project M and I know it's going to last.
I'd say you should seriously consider what you enjoy about the game, including against what opponents you have fun playing against, and then decide whether you are that committed to Smash 4 to spend over $100 for a one-game system.