WordPress plugins are great for adding extra features for your blog that does not come with the default WordPress installation. But if you have the same available via a cloud based web app, then you should consider the web app over plugin for the below reasons.
1. First thing user to consider before using WordPress plugin or web app is the security.
When you install a WordPress plugin (any plugin) you download and install lot of files to your server (mostly PHP files) that work from your server. The plugin is also universal and used in hundreds/thousands of other sites and the source code for those plugins are well known to the hackers including the vulnerabilities and how they work with different WordPress versions.
So there could arise security issues if the WordPress version is not updated to the latest and the plugin not updated to work with the latest WordPress version.
This is not the case with standalone web apps, everything happens in the server where its hosted and the source code is not shared.
2. If you buy WordPress plugins and if they are sold for an one time fee, chance are high they are not updated periodically and there could arise new bugs with ever changing WordPress landscape. Even if the plugins are updated frequently, there could be a lag/delays and all the manual effort to back up existing data and install the new updated plugin
3. With respect to scope of improvement and adding new major features, WordPress plugins could have a limitation that’s not the case with web apps. Web apps have all the flexibility you could imagine with respect to improving the product.
4. WordPress plugins also considerably slow down or reduce performance of your blog. Almost all the plugins add overhead to your blog and reduce performance even if its not directly slowing down your blog load times (which many of the plugins of course do by adding extra code/scripts to your pages)
You should also be aware that when connecting your WordPress blog to an external web app there is a connection channel established and your blog’s XMLRPC connection method is used for that. So its advised to connect your WordPress blog only with trusted web apps and periodically change your WordPress admin account passwords (like once in 3 months ideally)