Why are there so many different models and what the heck is the difference between them all? I have been reading about these but get confused about the differences. Can anyone that has any of these models please chime in here and let me know how you feel about the model you have and what or if you wish you had a different model and why? As far as I can tell, one of the models is weatherproof and one isn't. Some have bluetooth and some don't. Some have nav and some don't. I'm trying to decide but can't seem to get all the models straight. Which model is the latest and greatest? Thanks.