I’ve used the Gengo and the xLanguage plugin and can recommend the following:
If your Blog/Site will be identical in each language, meaning for each English post there’s a Spanish translation and visa versa, I suggest you use xLanguage.
If your blog consists of both English and Spanish posts that are different in content, meaning articles that are not just translations of each other, and want to display the articles in the alternate language if there is no translation, i.e. mixed language content on the same page, I suggest Gengo.
The major differences between these plugins are that Gengo adds extra tables to your database for the different languages. Posts that are translations of each other have to be linked to each other within the post (an extra option is added to do so). With xLanguage you may need to do some editing to your template files (a one time edit) but it works with adding the (X)HTML language attribute to your content. I find xLanguage easier especially if you want have the navigation and linkbars and things like date etc. to be translated as well. If you want a Spanish reader to see everything in Spanish and an English reader everything in English I recommend xLanguage.
With Gengo you have to choose a dominant language which will always be there in the links, dates and other more “static” content.