Go talk to him about a formal agreement...
If you really see no other way than to fund your own type rating then a formal agreement with your future employer is absolutely essential. Otherwise you risk spending a lot of money for nothing. There are plenty of unemployed Excel pilots out there already now, some of them with a lot of experience on type.
Coming out of the training facility will not leave you with a finished type rating anyway, but you will have to do your landing training on a real aircraft first. Without an employer who lets you do this on his aircraft, this will be very difficult and costly to achieve.
Regarding your initial question: My colleagues who fly the Excel more or less alternate between the two training facilities. Which one to choose is rather a question of getting a convenient training slot than anything else.
AFAIK none of these two facilities has the XLS+, so differences training will always be required. The best way to do this might be, once again, using the actual aircraft of your employer. So really, all this makes no sense at all without an employment contract of some sort!