I have never liked Movistar after a mega run in with different departments over the return of a radio phone box  >:(.However that being said,and I rather this was not the case,but I do believe they have a better mobile phone reception than Simyo (which uses the Orange network).This of course is only my opinion & based on my usage of both networks in different parts of Spain.
I always use my Simyo phone whenever I can, as of course PAYG wise I always find them cheaper than the basic Moivstar PAYG package.But,if you only want to use Simyo on the Delta & find their coverage to be good,then I would say go for it.    :)