Hello,

You can opt for commerce for your further studies, because you already have a base and there is Commerce and Accountancy optional for UPSC. You can choose this as your optional subject in UPSC. Choosing commerce will help you because you will be familiar with the subject.

You can choose BA also with subjects which are there in UPSC optional too. This will require more effort and time than commerce subjects, because you will not be aware of basics.

So it is advisable to take commerce. But it is just a advice, you can go for any.

Hope it helps.