Good answers already given, but in a nutshell, an SSD is quicker, more reliable and less likely to develop faults, as there are no moving parts involved.
The only downside is the SSD is more expensive than a traditional HDD.
If you can afford it, I'd def go for the SSD.
My HDD died on me a while ago and I had it replaced with a SSD. The laptop starts up almost instantly now and I can tell the difference when I'm using my laptop - it's def quicker than it was before :)