When comparing OpenGameArt vs Unity Asset Store, the Slant community recommends Unity Asset Store for most people. In the question“What are the best sources of royalty free game assets?” Unity Asset Store is ranked 2nd while OpenGameArt is ranked 3rd. The most important reason people chose Unity Asset Store is:
Those who have downloaded an asset can give it a rating (5-star system) and optionally a written review. Packages with at least four show their average rating, and total number of ratings. (Packages below the threshold show "Not enough ratings" instead.)
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Pros
Pro Committed to free/open licensing
All assets made available through OpenGameArt must be licensed under one (or more) of the following: CC0, OGA-BY, CC-BY, CC-BY-SA, GPL or LGPL.
Pro 2D and 3D art
OpenGameArt has both 2D and 3D art assets.
Pro Big library of varied CC0 assets
OGA currently has an impressive library of free assets, especially in the texture department.
Pro Supports asset requests
OpenGameArt has community forum sections dedicated to Resource Requests and Open Commissions.
Pro User review system helps gauge asset quality
Those who have downloaded an asset can give it a rating (5-star system) and optionally a written review. Packages with at least four show their average rating, and total number of ratings. (Packages below the threshold show "Not enough ratings" instead.)
Pro Excellent presentation, organization, and search
The Unity Asset Store is well-presented, with an intuitive category system and effective search. In addition, the requirements to place an asset on the store include providing several standardized graphical and descriptive elements so that individual packages match the same presentation standards as the store itself.
Cons
Con Mediocre organization and search functionality
The curator of OpenGameArt acknowledges that the tools for finding the assets you need are sub-par. Correcting this is a near-future priority for improvement, and as of September 2014 is the next milestone goal of the site's Patreon campaign.
Con Assets may have restrictive licenses
OpenGameArt hosts art under any of several licenses, some of which may not be suitable for commercial projects or for release on certain distribution platforms. Careful review of which licenses are compatible with your project is essential. The site's search facility allows filtering by license.
Con Unity3D license required to download assets
Though assets may be browsed through the web, they can only be downloaded from within a licensed and registered copy of Unity3D (Free or Pro). However, once downloaded, the Asset Store EULA does not prevent using them in non-Unity projects so long as those projects match the "games and interactive media" description.