I have a bottle of the both the regular and the XL.  The XL says its a few feet longer than the regular but haven't really noticed a difference (then again tend to only polish my spare ball, as with my style polish causes the ball to blow right through the break point).  The big difference for me is the XL includes a cleaner.  My Bean's secret sauce polish I do notice a difference with it being longer than the Black Magic but unlike BM its abrasive (mirror like polish) and significantly more expensive (but requires only a few drops to polish).  Black Magic won't change the surface grit.  Also have some Reacta Shine I use (also includes cleaner) which is also abrasive at like 1500 grit or so which works well but costs more than it should imo and is I would say in between the other two in length gained (surface grit change aside).  I guess they all have their place depending on what you are trying to do.  For example Bean's is the only one I would use to polish urethane but sometimes you don't want that mirror like polish on some of the reactives as you don't need that much extra length.

I found that Black Magic XL polish acted like a slip agent polish. It increased length but decreased backend and overall hook and decreased hitting power. So I stopped using it. I did not try any other Black Magic polishes.

I sent an e mail to Ultimate asking about the differences in their polishes  and was surprised to see that they are associated  Brunswick, DV8, and Radical. So I wonder if their polish is any different than the Crown polish ?