To be blunt............................
0.1% saving on €100k mortgage is €100 per annum or €8.3 p/m. Simple calculation.
That's 3 pints of San Miguel per month (at non Polaris rates 
). I drive everything down to the price of a pint at Northern rates.
The biggest things to consider on any mortgage are 'the fees'' and how they affect the APR (I see no Spanish Banks quoting an APR, you have to work it out for yourself), the exchange rate, the currency you will bill any renters (£ or €) and the currency that it will cost you in if you have to transfer funds monthly.
If an offer is over a longer term then the fees are spread over a longer term.............. work it out.
Via my own staff (financial coy) Spanish mortgage i can get 25 years with 10 years interest only at 5.13% discounted rate for 2 years (then 5.63% at current rates, ECB not expected to cut rates until Q4 this year), max term 25 years of which 10 years is interest only, This carries a 1% arrangement fee.
(The builders mortgage is 30 years, 5 years interest only, initial rate 5.75% give or take 0.1%)
This is also available to customers at the same rates (they won't discount for staff as the cost to us would increase due to UK benefit in kind calculations if they discounted it).
The European Central Bank (ECB) are holding out on rate reductions and instead are pumping cash into the financial markets to improve liquidity amongst lenders, whereas the Bank of England is trying to do it via interest rate reductions (in my opinion the wrong way) which puts more pressure on the £/€ exchange rate.
Catch 22 then as UK Spanish mortgage offers are slightly cheaper than Spanish Spanish mortgage offers, but, if you take the builders mortgage via Cam Bank then it cuts out €2,500 of registration fees for transferring the mortgage across Banks
It really is a case of work it out for yourself as everyones position is different.
12 months ago, i was going to cash in Isa's, convert to Euros and pay cash, however the UK stock market has crashed 30% on the stocks i hold and the £/€ rate has crashed 14%, so a double whammy. I will wait for a recovery in both.
I do believe the UK stock market will rise again, but may take a few years, but unless the bank of England take a different approach (every rate reduction puts more pressure on the £/€ exchange rate whilst the Eurozone hold their interest rates) to interest rates and liquidity then my preferred option is to put my mortgage into the same currency and country as my asset.
Horses for courses, and i still continue to think about it as it really is catch 22
Dave