If we get married in Mexico, is it considered legal in our own country? 
The ACTA DE MATRIMONIO (your proof-of-marriage certificate) is recognized by the United States, Canada, Francs, England, Scotland, Ireland, Germany and many other countries.

What is needed to have a civil (legal) wedding? 
The requirement documents for a civil marriage for foreigners in Isla Mujeres are: 
* A valid passport
* A certified copy of your Birth Certificate 
  (with the Apositille Seal) 
* Photo ID (i.e. driver's license)
* 4 witnesses with passports
* Your tourist card (entry visa) 
* Blood test results (can be obtained on Isla)
* Completed Civil Marriage form

What is the difference between a Civil Wedding and a Spiritual Ceremony? 
Only a Mexican civil wedding is considered legal. A spiritual wedding is a ceremony to reaffirm your commitment to your spouse. 

What is needed to have a Spiritual Ceremony? 
Just you and your spouse.