I don't think either one will be "quick" compared to doing it right. Routing the coaxial cable is the hard part. Drilling a 3/4" hole for the NMO mount on the cab roof doesn't take any more time than drilling the holes to support the L-bracket on the fender or headache rack.
However, given the two options, go for the headache rack.
There's a couple of good reasons:
1. Getting the RF energy away from the vehicle occupants. Running a 50 watt or more radio that close to the driver or passengers is not a good idea. Lots of amateurs do it anyway and will tell you "it doesn't matter", but on the professional side we do take steps to reduce RF exposure to vehicle occupants. Fender mounts on the front of vehicles are usually avoided for this reason. Also, in the olden' days, noisy ignition systems and radios didn't always play well together.
2. Putting the antenna down on the fender doesn't provide a very good ground plane. While it does have a ground, it's lopsided and will make for a funny radiation pattern.
3. The antenna on the fender often can result in unwanted reflections from the cab. If the antenna is too close to the metal structure of the cab, either on the front fender or on the bed rail, it can detune the antenna.
4. The higher the antenna, the farther off the radio horizon is. In other words, the higher the antenna on the vehicle, the better it'll work. While it may only be a 3 foot difference, there's no need to make things any worse.
5. Routing the coaxial cable through the pinch point of the hood, through a hot engine compartment where it may be exposed to chemicals, and then trying to get it into the cab doesn't make a lot of sense if you've got an alternate location that will actually work better.
Where the fender mounts work well is for people with garage clearance issues. Other than that, it comes down to what the owner wants it to look like, or is just being lazy (no offense intended). Sure, some are very happy with them even with the drawbacks. Not everyone has the skill to do a permanent install on the cab, however, it will never work better than a roof top install.
On the flip side….
Getting the antenna up high on the headache rack will put the antenna out in the open where it'll work better.
It will be at a higher risk for tree branch strikes, but if you do the install correctly and use a quality antenna, this shouldn't be an issue.
The ground plane is less than ideal, but it won't be lopsided in just one direction, IF it's mounted in the center of the headache rack.
Cable routing will require (maybe) some additional cable, but it will likely be easier. Look for access holes under the cab. All the trucks I've owned have had several.
As for the antenna….
Don't go cheap. You may save a few dollars with a Tram or Browning antenna, but you'll pay for that in the long run. I've looked at these antennas at trade shows, and they are not high quality. I talked to one of their guys, and he had very little knowledge about the antennas and had no clue who made them or where.
For a few bucks more you can get a name brand antenna that will easily outlast your truck. I'm running Larsen antennas that are nearly 30 years old.
Laird, Anntenex, Comtelco and MaxRad are all good brands.
Same goes for the mount. Get a good one and you won't regret it. Installed properly it'll outlast the life of your truck. I've got 25 year old vehicles here at work driving around with NMO mounts on the roof that still work just fine.
Choosing the correct antenna is important, too. Depending on what band or bands you want there may be several options.
For the fender or headache rack installs where there is a less than ideal ground plane, think about a half wave antenna. They do not require a ground plane to work properly, but they will work better with one.
Most other antenna designs require a ground plane under them in all directions for proper performance.
If you get the half wave now, you'll have no issues when you do transfer it over to the permanent mount on your roof.
If your needs require a dual band antenna, it limits your options. Again, stick with the name brands. Avoid the "Amateur" brand antennas. They are not better than the commercial antennas and are often marketed using gimmicks. You don't need gimmicks, you need a good reliable antenna.
And, in many cases, the commercial brand antennas can be cheaper that the gimmicky amateur/hobby grade stuff.