Showcase = Resource Manager + MediaGallary
That really isn't the case. All 3 addons do have some of the same/similar functionality (as do dozens of other addons), however, all have very different architectures and overall major purposes and functionality. Each has their own specific/unique functionality(s) that are not shared by the others as well.
1) Showcase ...adds grid view functionality in addition to all the functions in XF- Resource Manager
There are several
major Resource Manager features/functions that Showcase does not have (and vice versa). Even the similar features (like Custom Fields for example) are not the same between the two addons (in both backend management and frontend usage). Showcase does a lot more with Custom Fields than the RM does. As just mentioned, Showcase has dozens of features/functions that the Resource Manager doesn't have like for example: Custom Review Fields, light weight comments system, Enhanced Filtering System with several different types of filters, built in XFMG integration, built in CTA FT & Portal integration, built in [bd] framework integration, 4 different Layouts (Article View, List View, Grid View and Modular Sliders) (which can be set per category), Enhanced Rate/Review system w/review fields (per category), Search by Custom Field, Tags, Image/upload Permissions per user group, style overrides per category, Read Marking, View Counts, Custom Category Content (displays on category pages), Multiple Image Gallery layout types (to include the ability to integrate XFMG Albums) ....there is MUCH MORE And then there are OPTIONS (which is a huge part of showcase). The RM has around 20ish options settings (with none of them having category overrides). Showcase has well over 200 options with several of those having category overrides (to be able to set those global options on a per category basis).
2) MediaGallary and Showcase share 60% same functionality but Showcase seem lot of functionality with lot of flexibility.
Just as I explained above, there are several
major features/functions that XFMG has that showcase doesn't have (nor will it ever, because its not a "Gallery") and vice versa, there are things that XFMG won't ever have that showcase has because they are not features of a "Gallery".
If you want (need) an addon that allows you to upload files for users to DOWNLOAD, then you will want to use the Resource Manager. It is by far the best option available and is a 1st party addon.
If you want (need) a GALLERY, then you have a few options. You should at least consider the top two: XFMG (my personal choice, plus its a 1st party addon) or XenGallery (another excellent gallery addon).
If you want (need) an addon that has a TON of configuration options (both global and per category), that allows users to create content to showcase their items/products/reviews etc, is easy to setup/configure right out of the box with no customization needed, are OK with using a 3rd party addon, then you will probably like what Showcase has to offer.
It boils down to what your needs are (which you've not mentioned, so its hard to recommend). The advice I give (to anyone that asks me) is to use 1st party addons (RM, XFMG etc) when ever possible for obvious reasons (support, stability with core, always up to date with core, well coded, supported by 3rd party Style Developers etc...). If 1st party addons don't quite have all the necessary features/functionality you NEED, then find a 3rd party addon that comes highly recommended by the community through ratings or comments or word of mouth, is well supported and has what you NEED.
Hope that helps