Should I stay with Poormanscron or is it worthwhile to set up an external cron?
Unless I'm mistaken. Poormans cron significantly impacts page load performance when cron runs. I only use it on sites I'm quickly working with locally and have no intention of working on in the near future again.
Using crontab to run cron completely removes the detrimental performance of using poormans cron, apart from the subsequent page load having to re-build the necessary caches. Using crontab is the better option, it provides predictable behaviour, is more reliable, and has less of an effect on users.
You may also want to look into using drush to run cron.
As a side note, on the performance tab there is a setting for the 'Minimum cache lifetime' does not only affect the page cache but all caches. If a time is not set there then as far as I know all caches are dumped on each cron run. (There are one or two sensationalist blog posts which appear to say otherwise though, but if you look at the core code they are incorrect.) This is something else which comes into play with cron runs and performance.
Elysia cron: This is more cron on steroids, and is not a replacement for poormans cron, as the Elysia cron documentation suggests, you should have cron running every minute to get the most flexibility with it, which poormans cron is just not capable of.