Ok, in 50 words or less....
When Microsoft quit FSX, there were two different development paths, the 'normal' FSX, and ESP, which was a visual simulation platform, though based on FSX. For an intro to ESP, take a look at
Microsoft's initial press release here. Note that it gives one of it's uses as 'preparedness', which kind of explains Lockheed Martin's product.
Anyway, Microsoft sold the rights to develop FSX/MS Flight to Dovetail Games, and the rights to FSX/ESP to Lockheed Martin. So Prepar3d (pronounced 'prepared') 1.0 was kind of a combined FSX and ESP, rebranded as a military simulation platform.
As Prepar3d has been steadily updated -- we're now up to version 4.0 -- it is the closest thing to a modern MSFS simulator, had they continued development. You can only buy Prepar3d from Lockheed Martin's own site, and it is via download only. There are different license options, an Academic, Professional, Developer, Professional Plus, and Pro Plus Developer. The last two are if you are planning a military campaign, so don't apply so much to us...
The license for people like those who hang out on NZFF would be the Professional, at US$199, which is expensive -- they will add GST, so it becomes more than NZ$300. So a lot of people buy the Academic version (US$60). You will find that a lot of sites ban discussions of the license, as it 'encourages poor moral choices'. or some such rubbish. However if you are buying software from a US defence provider, then maybe morality is not your main decider...
So, I recommend the $200 version for those who can afford it, and the $60 version for those who can't. I use the monthly developer license, which is simply that, it costs me about $16 a month, which makes it about NZ$200 a year.
As I said, we've recently had version 4.0 released, and this is the version to go for, although you can still buy the previous versions.
Most major developers who support FSX also support Prepar3d, including Orbx. And me, although still no installers for 4.0, they're coming... There is less and less compatibility of older FSX aircraft as Prepar3d is developed, so if you can find Prepar3d versions of anything you own, then you are better off. So don't expect to be able to dump all your aircraft into Prepar3d, it won't work well.
Whether or not it's worth it -- most people who try it, love it, and many quickly and completely make the switch. It is a more modern product, continually in development, and now 64 bit, which removes the memory management issues with FSX+addons. If you are unsure of whether it is for you, I recommend buying a single month's developer license, and trying it out for yourself. This will cost $16, as I said, but is a cheapish way to trial.
In conclusion -- have you looked at X-plane?:) Cheaper, 64 bit also, continually in development.. what more could you want? (That's only partly a joke...)