A criteria I always find key for hiking trails, is how many are actually present in my own city of Cape Town, especially the smaller off-the-beaten-track trails. Both of these apps cover a good many of those, with AllTrails coming out a bit ahead though for my city.
Although most hiking trail apps offer free apps, there is usually a pro paid version for additional extras like ad-free, offline maps, alerts to going off-route, live tracking/sharing, etc. In this regard, AllTrails is a bit costlier at $30 per annum whilst Wikilocs is around $10 pa (with shorter period options too).
AllTrails does also offer slightly more in terms of search options around difficulty, length, elevation gain, time, route type (loop, out and back, point to point), star rating, activity type, attractions (e.g. cave, forest, historic site, waterfall, etc), suitability (e.g. dog friendly, wheelchair friendly, stroller friendly, etc), and trail traffic. Wikiloc is really only by activity type, distance, elevation, difficulty, loop trails (Y/N), how long ago recorded, and month recorded. Unique to Wikiloc is a "passing through" area search to get trails that will pass through a specific area.
GAIA GPS is also a popular app but firstly not many local hiking trails for me, and it's $36 pa full membership is just a bit pricey for me. All three mentioned also work with Apple Watch, and I used my Watch to monitor our route progress with AllTrails.
Cairn has an interesting option that also maps crowdsourced cellphone reception areas, which is useful to know what sections of your trail will have reception. You'd think they'd all have this.
See AllTrails: Trail Guides & Maps for Hiking, Camping, and Running | AllTrails
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