Hi Diane,
From reading your replies to Frances and others I'm not sure whether you know that the wills and parish registers on film are scans of the (mostly) handwritten originals, or in the case of the par regs, scans of the official copy made in the early 1900s. These films are much the same whether you look at them in the Manx Museum library, or at a LDS Family History Centre (sometimes a different - more recent - filming by the LDS). This is why Frances suggested looking at as much as possible beforehand rather than wasting precious time in the IoM. Do they have FHCs in Spain? If not, you have no choice, and my suggestion is to get photocopies whenever possible, rather than having to transcribe as you go. I have been in the same situation with limited time in the IoM, and later regretted not getting more photocopies of documents held in the MM which are not available on film. Look at all the indexes there before doing anything else - you may find charts, photos, etc., there for your families. There are also records not available on LDS film, like marriage agreements and land records (see MM indexes). Also don't miss the bookshop.
Although I enjoyed the Manx Museum, the General Registry (civil regt certs and more recent wills), and the FHS library (charts etc. deposited by members - take photocopies - and they opened especially to fit in with me), I spent about half my time looking around churchyards, farms, and towns where my ancestors had lived. I was extremely lucky in being offered these trips, because I had never met anyone in the IoM other than by correspondence, and had expected to hire a car. People couldn't have been more helpful, and a cousin I had "met" only a few months earlier insisted on taking me home for lunch, and her husband patiently drove us all over the island. She also introduced me to a number of well known genealogists on the island who were all most generous and hospitable.
Before arriving I had told only the IoM FHS that I was coming, and several committee members went out of their way to make sure I saw as much as possible in the time available (I hope you are a member?): http://www.isle-of-man.com/interests/genealogy/fhs/.
Sue