I have completed BE(mech),is doing MBA in foreign country is better or clearing CAT and doing MBA in own country is better

See it's all your choice whether you want to pursue MBA degree from foreign or from India, only thing is you need to do is swot analysis of the options available in your plate.

The value of MBA degree depends on the college from which you will be pursuing MBA degree. If you are not able to get college or universities like Stanford, HEC Paris after clearing Gmat and the whole rigourous process then it won't be much valuable as comparison to doing MBA from top institute within India like ISB Hyderabad, IIM A,B,C.

Before making any decision you need to compare the college in all terms, if you are able to get a decent foreign university and instead of ending in top Indian Institutes you have an option of going into new IIMs then foreign university will be a better option as it gives you an international exposure.

One more thing you need to keep in mind is that the top foreign universities ask for atleast 3 years work experience. Even if you are selected this year still you will need to work for 3 years and then only you can pursue your MBA degree which is not the case in Indian Institutes.

As an advice I will recommend you to not to make any decision right now and give your best in both the exams GMAT and CAT. Later after going through whole process you can make your choice according to universities available in your plate.