How do I show posts from another wordpress installation?

Non-Multisite setup:

You need to create another instance of the WPDB Class.

$newWPDB = new wpdb('Username','password','database','localhost');
$rows = $newWPDB->get_results("you-query-here");

On WordPress Multisite:

Does having WordPress MultiSite make things a lot easier or should we
stick with single installations?

There is the switch_to_blog() function available. You will need to restore_current_blog() after getting your results.

Note that you could cache the results, for example using the Transients API, to reduce server load on future requests.