I do fly a lot, so, for the un-initiated, I hope I can simplify this (I've had both for a number of years, and each keeps evolving, so some of the details here may change, but this should be general enough to see the difference):
- Pre-Check is a program offered by the TSA that makes it easier to get through airport security. As a Pre-check customer, I get shorter lines, less invasive screening (e.g. I don't have to go through the naked body scanner, remove shoes, remove belt, take PC out of the bag, etc.). Instead, I keep on most everything, put my bags on the conveyor, and walk through the magnetometer. In short, it's a better way to get into the secure are of the airport so I can fly.
- Global Entry is a program offered by the CBP that makes it easier to get through customs when re-entering the United States. As a Global Entry customer, I get shorter lines, less tedious interview process (e.g. an interview with a CBP officer who is trying to identify who I am and query me about any number of things + a second line after leaving the baggage area). Instead, I put my data & biometrics into a kiosk and have a very brief face-to-face with a CBP officer, and if I didn't check bags (which I usually don't, even on international trips), I can bypass the baggage area and the second line altogether. In short, it's a better way to get into the country so I can get back home (and re-arm, of course!)
- If you get Pre-check, do you automatically get Global Entry? Nope...these are two programs offered by two agencies, each with a different purpose.
- If you get Global Entry, do you automatically get Pre-Check? Nope...but it does help, since you will have already received your 'Trusted 'Traveler' number. But you still have to apply.
- Are there other programs? Sure, CBP has several other related programs, such as FAST, FLUX, NEXUS, SENTRI, SES, Smart Gate and STEP. Some of those (e.g. FLUX, SES, & Smart Gate) offer us who frequently travel to/from the Netherlands, S. Korea, and Australia to have simplified entries into those countries. NEXUS and SENTRI simplify entry between US, Canada, and Mexico for those who do so regularly.
- Global Entry costs $100. Some frequent flyers get reimbursed for this (see last note below).
- Pre-Check costs $85. Global Entry members don't have to pay this.
- Global Entry is accepted at around 45 airports right now.
- Pre-check is accepted at around 115 airports right now, but only if you're flying one of the accepted airlines. For instance, when I fly United, I get it most of the time, but when I jump on a Singapore Airlines or Lufthansa flight, I don't.
- If you have Global Entry, is it always available? Almost always. Rarely have I seen the kiosks closed, and there is no random selection process. If the kiosk is open, then I can use it.
- If you have Pre-Check, is it always available? Most of the time, but not always. As I mentioned above, if you're flying the wrong airline, your out of luck. But also, the process routinely selects you to go through the full screening. It rarely happens for me, but sometimes, it does. Of course, they tell you up front if you've got it or not, because it is printed on my boarding pass before I ever leave home. When I don't have it printed, I leave for the airport a bit earlier to make sure I have time enough to get through the full screening process (complete with opt-out of the full-body scanner and the requisite pat-down after).
- Some airlines will pay for their most frequent travelers to participate in these programs. For instance, United will foot the bill for Platinum, 1K, and Global Services members of their frequent flyer program for Global Entry.
So, to get back to the OP's point...
If you ever do any international travel, I'd get Global Entry. From a financial perspective, it makes sense, because then you don't have to pay for Pre-Check, and you could get both programs for only $15 difference.
If you are a frequent flyer with high enough status to get it reimbursed, get Global Entry. That way, you can effectively get both for no cost.
If you never travel internationally, and you don't have high enough frequent flyer status, then Pre-Check is still worth it, if you fly several times per year (you have to weigh the cost-benefit analysis of the $85 vs. the hassle and time at the airport for yourself).
On a side note, the CBP officers I've met have all been very professional and good to work with. I see mant of the same ones on a regular basis as I enter at Bush airport in Houston, and I'm overall quite impressed with them. To see that organization come up with something as logical as Global Entry for regular travelers doesn't surprise me one bit. (Note: this is not an original program...when I lived in Singapore, every legal resident of the country (me included as a foreigner!) were able to use a very similar program at their border using automated kiosks.)
Many of the TSA folks I've met have been quite good - and some have been a bit spotty - but I'm definitely not a fan of the decision-making the TSA does at the higher levels. So who'd ever think the bosses at TSA would come up with something good like Pre-Check? I guess even a blind squirrel finds a nut, every once in a while.