Should I withdraw a submission?
This is clearly a judgement call and I can only offer opinion. But if you withdraw from the conference at which it was accepted you won't be able to resubmit there if you are rejected by the other. This narrows your future options for this paper. 

I'd therefore suggest that you stay with what you have (a bird in the hand is worth two in the bush). 

But this is just one paper. Write more papers and expand your knowledge about good places to submit them. One paper is just one paper. It isn't your entire career.