Welsh slate has been used for generations in Ireland and it is extremely strong, durable and has a very distinctive color. Welsh slates tend to stand the test of time and it has an excellent reputation. This is due to it having a unique blue hue and also how it is waterproof and highly resistant to acid, alkali, UV light and temperature change. Welsh slate can last an extremely long time and can even be removed redressed and reused allowing it to last up to hundreds of years.
The quality of Spanish slates are a lot more varied. There are a lot of Spanish quarries and therefore a lot of Spanish slate is imported every year and some of it is fairly poor quality. Some slate roofs that have bee installed using Spanish slate have failed only decades down the road but this depends entirely on the quarry that mined it and there are good quality Spanish slates you just need to be a bit more careful when purchasing it. Simply make sure it comes from a quarry with a good reputation.