Your position is very much typical. I tell you why I said so. You are graduating in Commerce (Accountancy) and you were in science background at +2 level whereas you are in dilemma of Bank PO or SSC which requires mainly mathematics background. However, since you have approached ISC, you will have many tips and you are at liberty to choose the best one you think out of many.
For both the competitive examinations, one should have the best skills in the subjects of mathematics and reasoning apart from GK, aptitude and general English. If you have command over Maths and English, half of the job in your pocket and rest half is others for any kind of competitive examinations. Since you are so particular about bank PO or SSC, with my experience I tell the advantages and disadvantages in the both jobs.
If you are selected through SSC, you would get right from clerical job to middle level jobs i.e. Group.B posts. But once selected through SSC and posted in a isolated departments, there will be no chance for career improvement and you have to be in the same post for years together. I don't say this applies to all the posts selected through SSC. But there is one kind of advantage that though the central govt. post is liable to be transferred to any where in India, upto the level of officers, there won't be transfers in place of posting.
If you are concentrated and selected for Bank POs, that will be a direct entry into the officers category at the initial stage of service and in the banks particularly time bound promotions and time bound pay revisions are in existence. I can say the career advancement would be at a rapid speed. However, the work load and pressure is very much high as I learnt and all the advantages you would get with regard to career advancement, would be yours once you are ready to take the pressure and workload.
Finally I would say, just prepare for all the competitive examinations with full of your strength and a wonderful job would be yours.