The three big trails are really different.
The Appalachian Trail
The AT (Appalachian Trail) is very social. It has shelters. More than 250 shelters averaging about eight miles apart.
The shelters cause hikers to cluster (even if they only camp near them). Shelters usually have a nearby water source and a privy. They also usually have a bear box or cables for safe storage of food at night.
The trail follows ridge lines and the tread (the footpath) is really often very rugged with roots and rocks and a lot of vertical per mile. Trail erosion is a real factor in places.
People avoid alternative routes which have blue blazes. The main trail is white blazed (has white vertical rectangles painted on trees).
Finally, resupply is easy, often only 30 or 40 miles down trail and often can be done at low cost at a Dollar General or similar store. There is a well developed culture around hikers, well developed support services and hostels.
It is possible to slack pack the entire trail or to spend virtually every night off trail in a bed. Books have been written about how to do both.
Both the northern and southern trail terminus points are close to roads and are fairly easy to access. Both terminus points have day hikers hiking them.
The AT is a temperate rain forest and it does rain a lot. “No Rain, No Pain, No Maine” is the catch phrase. On the other hand, micro spikes, ice axes and crampons are not generally used on trail.
The Pacific Crest Trail
The PCT is longer than the AT and people usually take a month less time than it took them to hike the AT to hike it. Most of the social aspect is at town stops.
The trail is really sweeping with great views. It has very few alternatives, but people do not have any qualms about hiking them. For safety there are times the alternative routes are preferable.
While the southern terminus is easy to drive to, the northern terminus is a hike in and hike out over several days.
There are large sections with extensive fire damage on the PCT and there are usually closed sections and temporary reroutes due to fire.
There are a few long water carries and a few places water caches are landmarks. I never dry camped on the AT. I did on the PCT.
There are also some high chaparral stretches.
Resupply is often further than the AT. The average is 74ish miles. Often resupply occurs in a resort town and can be pricey.
There are maybe two or three shelters, none of which are really ones you would want to camp at. At times the trail runs through abandoned camp grounds.
The trail is graded. The PCT is also shared with pack animals which are banned from the AT. It has switchbacks but you are much more likely to cross snow fields and will need microspikes or crampons and an ice axe.
Blazes use the PCT logo and they are intended to be unobtrusive. While the trail on the AT was obvious (except once or twice due to heavy leaf fall) it isn’t always quite as clear on the PCT if there is snow.
You are also going to ford deeper water. The AT has all of its fording in the 100 mile wilderness. The PCT has fording more scattered out.
Both have trail families as a common feature.
Happy6 and I encountered a lot of blowdowns on the PCT. Pretty much no road walking unless there was a fire bypass. The PCT needs a lot of maintenance but it is generally clear where the trail is.
For both the AT and the PCT, FarOut (the app) on your phone is really enough.
The Continental Divide Trail
The CDT has most of its social features on the shuttle to the south terminus. There are occasionally other social spots, but often socializing is sparse or random.
Services are often significantly less.
There are a lot of long water carries and places where all the water comes from caches. In fact, your first four days on trail will probably all be using water caches.
Not to mention the cow water tanks and troughs.
Resupply is often a hundred or more miles from one to another.
The CDT has a lot more large wildlife. Wild horses, herds of them. Many Antelope. Elk. Grizzly bears. Moose. (Ok. Other trails have some of those but the CDT is where I saw them).
The CDT is a trail for carrying bear spray in places.
The CDT’s trail is winding and has a lot of walking on dirt roads. It also is “nothing” but alternative routes. Sometimes the official route has no tread, maintenance or blazes while an alternative is blazed, cut and maintained.
Much of it is also not blazed.
It has places where there is no trail tread —where there is literally no footpath. It has places where the trail is a bushwhack and everyone seems use dirt ranch roads that are in the same area.
As for fording, especially with the Gila you can expect to ford over a hundred times.
While it crosses some true deserts it also has monsoon season in Colorado with some real thunderstorms and hikers do hit snow and ice. Happy6 and I got “lucky” because of the way the weather bounced us around so that we missed a lot of snow, ice, and storms.
Avenza, COTREX (a free app for Colorado trails), Garmin, FarOut and CalTopo all came in handy and at least once were all wrong. Happy6 also used OnX Maps. Avenza ha the Ley Maps available for free through the CDTC.
That captures the difference between the trails.