CAT vs GMAT is a tough choice and a lot of factors should be considered to make it a good one.
GMAT is considered easier to crack than CAT based on very low conversion ratio in CAT. However, GMAT is much more expensive than CAT, both in terms of application as well as college fees.
Also, Indian B-Schools are more inclined towards lesser work experience of candidates. In contrast, B-Schools abroad have minimum work experience requirement.
I personally feel that IIMs, XLRI, FMS etc. are more suitable and make more sense if you want to remain in India in the initial years of your career.
So, based on the above scenarios, you need to make a choice as per your circumstances.
Just sharing an interesting fact. Many of my friends who could not clear CAT even after 3-4 attempts finally took GMAT and got into some decent colleges abroad.
For preparation, the content of both the exams is mostly similar however exam pattern is quite different. So, basic preparation can be same for both the exams but you need to subscribe to different test series to practice for the actual exam.
I would suggest to identify your situation based on the scenarios I mentioned earlier and choose to take only one of the two exams. No need to ride in two boats. Going for both the exams will only distract you.
However, if you want to take both the exams for the sake of keeping a back-up, you should first go for CAT because it is a fixed date exam. Once you take CAT, immediately start preparing for GMAT. Having already prepared for CAT, you have little to study and just need to practice a lot as per the pattern of GMAT. Once you get your CAT result, schedule a date for GMAT giving a time gap of about 4-6 months.
Meanwhile, if you clear CAT, you will have to devote a lot of time for interview preparation. So, you can plan your GMAT preparation and date of appointment as per your interview schedule.
Below is information about good resources for preparation of both the excams-
For CAT Preparation, you can refer to my article here- How to start preparing for CAT (IIM)