IMHO, hardware PhysX is not a must, yet something nice to have, but not necessary. Many nay sayers are right, not many games support PhysX, and those that does are likely not fun to play. However, hardware PhysX is not an unit within the card, but is a method of utilization of those CUDA cores, meaning that nothing is wasted on non-PhysX based games. What gets confusing is PhysX works regardless of the brand of video cards. Since games are designed for players and not video cards, the core mechanics of any games will work on both camps, so you won't miss a bit. At most, you lose some eye candy in exchange of better framerate.
Having said that, Nvidia video card is not more expensive then AMD cards. You don't pay more for hardware PhysX, you get it if you have a Nvidia video card. Check out Anandtech benching and then check the price of the cards and you will realized that more you pay, the better card you will get. If that card is Nvidia, then you are open for hardware physX. If the card is AMD, then you are open for triple display with a single card.
There are no down side of hardware PhysX, so getting a Nvidia card do give you a possibility of having better visual experience on say 1 or 2 good games for the live of the card. If hardware PhysX isn't strong enough motivation when it comes to selection, then pass, and you can still be able to enjoy PhysX with other cards. Note that other than PhysX, Nvidia's card are also better at tessellation. As of now, nay sayers claim that current games don't need those level of tessellation. If tessellation does get adopted in general, it will only get more demanding in that department, meaning that with a Nvidia card, it may age a bit slower than an AMD card. Also, with red/blue 3d glasses, you can also experience 3D with a Nvidia with no other extra equipment.