I'm in exactly the same boat as you my friend. I got a PS4 two days ago, and I took the leap of buying Project Cars as my first game. You are probably concerned mostly, as I was, with getting out of the GT/Forza comfort zone and trying something completely new to you.
In response to that concern, I will say that Project Cars is, in a word, unbelievable...and I only play with a controller, not a wheel. The first thing that you will notice is how much MORE the gameplay is. I know that is terrible grammar, but it is true. The detail and texture of the graphics, the intensity of the feedback, and the sound, oh the SOUND! The sound is staggeringly good, it's sublime.
Yes, there are things that will take some getting used to reading the feel of the cars when using a controller, and I highly recommend that you watch this video: when setting up your controls.
Additionally, I use the right thumbstick for throttle and brake, and manually reformatting the button assignments was difficult to do right out of the box. (A word to the wise - if you do want to reformat the button assignments, make a note of which functions are assigned in the default setup and make sure they all still have a button assigned to them when you are done reformatting. Otherwise, you will get a 'not all required operations are assigned' warning.)
Overall though, look at it this way: play Project Cars for a couple of hours and then go back to GT6. I'm sorry to say this because I love GT6, but you will be disappointed by how much LESS GT6 is and, as I said above, you will be so impressed by how much MORE Project Cars has to offer. Do your research, become informed, and you'll make the right choice for you, don't worry.
P.s. #1 - Make sure you download Update 7.0 as well as all of the free DLC cars.
P.s. #2 - Tuning the cars is a much, much more involved process in Project Cars that in GT6 due to the sheer quantity of settings that can be altered. However, there are innumerable posts and videos out there to guide you through the process, and you might just enjoy the opportunity to learn how much more there is going on in a car than is presented in GT6.
P.s. #3 - It is true that Project Cars lacks the 'save your credits for upgrades or new cars' format to the career mode, and for some people this will seem like a bigger loss in the experience than it does for others. My personal view? Project Cars focuses on the racing, and only the racing. Since that is what I wanted the game for, I'm not bothered at all.