I've used the lounges at both, although BLR was a long time ago (not long after the new airport opened). Both are fine, and you get a much more choice of food in the DXB lounge that you would in any BA lounge.

At DXB, which lounge you use will depend on which concourse you flight is departing from. I think there's a Marhaba lounge in each terminal, plus a couple of others.