The one you currently have states that you are on the register of EU foreigners resident in Spain.
The Residente Permanente ones don't say you are a EU foreigner.
Now, it probably won't make any difference (we hope) when the UK leaves the EU as the EU rules state anyone (EU citizen or not) residing legally in an EU country for over 5 years should be regarded as a permanent resident in that country although non-EU residents are to obtain a card stating that.
The procedure for getting the permanente one is exactly the same as getting the original card and you simply tick the bit at the bottom under Residente Permanente marked durante cinco anos.
Sounds easy but, in this area at least, getting an appointment at a National Police station or a foreigners' office is a bit of a long wait with none available in our local area (Elche, Torrevieja, Orihuela Costa) until April at the earliest.
Bit of a belt and braces thing to get the permanent one I think.
From the EU site:
Towards the end of their 5th year of continuous legal residence your non-EU family members should apply for a permanent residence card in the EU country where you live.
The permanent residence card is often issued free of charge (or for the same price as identity cards for nationals). It should be valid for 10 years and is automatically renewable without any condition or requirement.
When applying for their permanent residence card, your non-EU family member must give proof that they've been living legally in the country for 5 years - for example, using a valid residence card issued 5 years ago.
The above is for non-EU citizens.
And for us EU citizens
As an EU national, you automatically acquire the right of permanent residence in another EU country if you have lived there legally for a continuous period of 5 years. If you fulfil this requirement, you can apply for a permanent residence document, which confirms your right to live in the country where you now live permanently, without any conditions.
This is different from the registration certificate which is compulsory in many countries. The permanent residence document is not compulsory.
I suppose it could be argued we won't be EU citizens after March (maybe).