It depends on the condition of you existing roof. Tile, in general, adds more value to the home. Having said that, I have seen some existing conditions that are bogus and need to go. For example: if your tiles are fastened to the roof with mortar only, and no mechanical fasteners, the mortar begins to seperate at some point, and during a storm you might produce deadly projectiles of tile from your roof. There may be underlying conditions of poor flashing and/or rotted wood that are not visible to you, but can cause big problems.

I know an awful lot of people that are scrapping the aging tile roof in favor of shingles for matters of soundness, safety and cost. But I can't see your roof from here, my friend, so you will have to make the call!
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State-Certified Building Contractor since 1978