I would suggest you to pursue B.Tech course in Computer Engineering after completion of your diploma. My recommendation is based on the following factors-
1) B.Tech in Computer Engineering takes four year in its completion for an aspirant having passed 10+2, but in your case, you would save one year by virtue of having possessed the diploma in Computer - engineering.
2) Persuasion of B.Tech in Computer Engineering from an established institute would enhance your job prospects in the upcoming time.
3) You will have strong fundamentals in the different languages such as Java, Oracle, C++, SAP etc and you will remain in demand for the same reason.
Now let us discuss the prospects of BCA course. In your case, you would be offered the admission in second year of BCA meaning there by it would take five year full time for the completion of BCA followed by MCA. Acquisition of MCA would be essential so that your degree is treated at par with a degree in Engineering. In the former case, you have to study merely three year in order to get a degree in Engineering. Thus saving time is an important consideration by opting to B.Tech degree.