Hi Maxed-out,
First of all, all the best for your future employment.
Regarding your question, my answer would be none.
FSI is really good but they teach the Excel, CAE are OK (most of the instructors are really good and some have no idea and teach you a different aircraft) but they teach the XLS.
The XLS+, being a variant of the C560XL rating, is quite a different ride. While all systems are more or less the same, (compared to the XLS,) the office is completely different (proline 21 and Collins FMS compared to the Honeywell Primus 1000+ and universal tamagotchi). If at all possible try to go to Wichita, that's the only XLS+ simulator around.
If your financials do not permit it, then I would suggest to go to CAE.
Please google a picture of the front office and look at the differences between the XLS and XLS+. I think that the only point in common is the Yoke and the APU control... Try to get your future employer to pay for the differences training.