Selecting a URL shortener is at your personal preference. In practice, all URLs of any kind are shortened by X.com’s own shortening service at http://t.co. Your shortened URL may display instead of the t.co URL in the web site or in a client; but the URL has still been shortened under the hood. If you do not shorten your URL, it’s more likely that the t.co URL will display.
This is only true on X.com; Bluesky and Mastodon do not contain internal URL shortening.
Advantages to Shortening
The advantages to shortening your URLs are mostly about statistics and branding.
Branding: Using a service like YOURLS or Bit.ly, you can define a custom short URL domain that relates to your web site. For example, I could use http://jdlsn.com/ to produce short URLs for http://www.joedolson.com. There’s no character savings, since the t.co shortened version of my URL will be 23 characters no matter what I do, but the jdlsn.com domain is slightly more likely to be displayed to the user.
Statistics: Using services such as Bit.ly, or YOURLS, you can gather statistics on the clicks for each link you’ve submitted to each services. These aren’t gathered by XPoster, but by the services used to shorten links. This can be helpful for determining how many people are actually clicking through to your site when you post. Your choice of a URL shortener has no impact on how long your URLs will be on X.com or how many characters you have available to write your update.
Disadvantages to Shortening
Because shortened URLs mask the target URL, some users may be reluctant to click on or share your links. Additionally, if a shortening service goes down (either temporarily or permanently), your links will stop working while the service is down.