I would second learning QGIS instead of Arc.
I use ArcGIS, but... I'm not the one paying for the licence. Arc can be glitchy, annoying, and makes me swear a lot. If you find yourself needing features that Arc handles better than QGIS, then I'd consider the Arc licence. Note that the ArcGIS licence is pretty limited for the "Basic" level, and even the "Standard" and "Advanced" levels depending on what you are doing, you end up adding on other expensive licences for toolboxes, and it's really just bloody expensive. For both programs, depending on what you end up trying to do, learning basic Python may eventually become helpful. You will probably also need a subscription to some sort of imagery provider, although, again, sometimes there are free airphotos available (which are far better than satellite).
That being said, a professional GIS software makes standardizing workflows so much easier, and makes professional looking maps far simpler. It's very quick to make a very similar map for two different areas, and use the same symbology, formatting, etc. I can knock off 3 versions of the same map area within a few minutes of each other, with different symbology, to see what the person wanting the map finds easier to read, or to highlight different things. Often, exporting the map is slower than making the changes.
GIS stands for Geographic Information System- it's basically a giant database with spatial data attached. The real benefit is when you start using it like a database instead of just a tool to draw pretty pictures on, and, IMO, this is the most fun part of GIS.
Being able to create, quickly, for example, a 50 m buffer around a stream for an environmental setback, show the contours of the land, and draw lines quickly that go perpendicular to contours can be really useful. Maybe there's a tap, and you can create a buffer showing what area is reachable from the hose. Maybe you colour south facing slopes blue, and north facing slopes red. Or maybe show what areas do not have a nitrogen fixing tree within a certain distance. Sky's the limit, really, when it comes to the fun analysis you can do. If you want to see all birch trees that are further than 10 m from the closest
apple tree that are ALSO less than 5 m from the nearest maple... you can do that. Don't know why you would, but you COULD.
Much of the US has really good contour data available for GIS. I think the USGS has most of the US data? Here in Canada, being able to pull in information regarding wetland areas, protected areas, lot boundaries,etc, is very useful to know what is and isn't permitted. I would take a look at the places you most often work in and see what GIS data they have available - you may be surprised, and that data has saved me a ton of work in the past. Data is often available from all levels of government - municipal, county, province/state, federal. I know my employer has sold as an expensive add on service, something that took me 30 min to do in ArcGIS with free data, and another hour to make it look pretty.
You can buy for some money (possibly sub $200 and from Garmin?) a handheld GPS that is far more accurate than the iPhone gps and bluetooths the location to the iPhone. That's what I would personally use. For a ton more money ($10 k + yearly license?) you can get a survey grade GPS. You can also rent survey GPS units, but they are likely way beyond your requirements.