No comparison unless you are happy with only using 2 different map layers (Topo/Sat view) OnX was crazy buggy until the recent update. I couldn't even stop recording a track. You still can't directly import GPX tracks into the app, have to do it online. I'm the Gaia Guy in my circle of friends. With that said I constantly look at other apps to see if anything else is worthwhile. I have yet to find one.
OnX has a filter for trail type. Dirt Bike Trails, 50" Trails, High Clearance 4X4 trails, and Full Width Roads. My guess is that is a filter from MVUM data and possibly National Forest Road data. I personally don't find that very useful. We have thousands of miles of dirt in AZ and very little would be categorized as for dirt bikes only. In a small sampling of data turning the 4 options on and off made no changes to the trails highlighted in my area. In Gaia GPS you can tap on trails and get the same data. A little less convenient but again I don't find that feature of OnX very useful.
Widest map you can download in OnX is 150 miles wide at low resolution. Gaia GPS has no restriction like that other than a 10,000 tile limit set by some map providers.
OnX has a long way to go.
Here are some screenshots from the online dashboards vs apps. Gaia can show all of the same info. It is just broken down into different layers which you can turn on and off. I have included an example of clicking on a trailhead to show pop-up data. Gaia has the correct link. OnX sent me to Yosemite park. Hmmm
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OnX
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Gaia Topo layer
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Gaia with recreation layer turned on and tapping a trail head
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OnX. Tapping the Additional Info Red link sent me to Yosemite instead of the proper website.
I would save your money.. Maybe look at OnX in a year. For the same pricing you can get over 250 map layers with Gaia GPS instead of 2 plus a much more mature app.