If you are set on staying in the Bathmate line my suggestion would be to go for the x40 . You will girth out the x40 pretty quickly and the gaiters will wreak havoc on your shaft. My thoughts on the X-40 is that experience is very helpful in using a model that big and starting with it might lead to alot of frustration. I say this because I am around 6in midshaft girth and still have trouble using the x40 even. My nuts want to join the pumping party. The X-40 for me is beyond what I could use.
Hope this helps