XAT

The syllabus of CAT and XAT are very similar in terms of Quant and Verbal.
Apart from this, XAT has GK and essay sections, which are taken into consideration in later stages once you clear cut-offs.
Decision making is the make or break section of XAT, CAT does not have any such section
Also, XAT introduced a new scheme of negative marking from 2016. This scheme had negative marking for incorrect responses and also for questions not attempted (>13 questions only), i.e. you could choose to not answer 13 questions. After this, you will start being marked in negative for every question you don't attempt. This is unique in XAT. This is not something to worry about as negative marking is very less probably insignificant in the final analysis.
Lastly, CAT is a computer based test and XAT is a pen-paper based test. Though the level of XAT has declined over the years, still XAT remains the only gateway to top tier institute of the nation which is XLRI.
XIBM, S P Jain, XIME are few institute which accepts XAT score. Every year, CAT has been under scrutiny for the way it was conducted their examination. XAT remains ethical in that sense. I suggest you to prepare well for CAT and probably in last month you can enrol for a decision-making course.