Why should I use a Feature Service instead of a Map Service in ArcGIS Online or Portal?
Layer hosted on AGOL are all FeatureServices you don't have a choice of MapServices.
Most of our AGOL Web Maps are based in Map Services.
What will be the pros and cons of changing them to use Feature Services?
FeatureServices just add the ability to edit, which is not possible with a MapService.
What do you use in your Organization? AGOL Web Maps pointing to Map Services or Feature Services?
"Besides editing, feature services also provide the option to dynamically set symbology for features at the client side. Map services essentially provide images pre-symbolized at the server side - either as pre-cached tiles or as dynamically generated images made from an exportmap request. Map services can be cached in order for data to render quickly. That's why basemaps are generally cached map services, while feature layers that render on top of basemaps can be either from a mapserver resource or a featureserver resource."
From Feature Service vs. Map Service
This may be a reductive response but who should do the work? Should your ArcGIS Server site instance handle repeated requests for images via the /MapServer endpoint? Or would you like for the geometry to be drawn client side and the user's browser/application to be responsible for handling the processing/rendering of the data?
In my experience, it has been the reverse - featureServices are too slow for almost all my use cases. I usually have a parcel polygon layer with hundreds of thousands of features in it. Despite scale dependency, web Mercator fGDB or SDE with compress and spatial indexing up to date, it still lags, even with hundreds of gigs RAM dozens of cores and n+1 pooling. I am very excited for when they add feature service 'tiling' to Portal 10.8 or thereabout. (It was tentative for 10.7.1 or a release soon after per Dev Conf) It will vector tile in to the mySQL like how it currently does on AGOL to generalize geometry. That feature alone has sold me on installing Portal. We have a need, a need for speed. Good point regarding mobile CPU power. I will be excited to see feature service tiling broadly adopted.
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