Playing one table, you can expect your ROI to be better, but your going to experience more variance. The rake is better, but the competition starts to become a little better also.
By playing multiple tables at a lower limit, your ROI should be a little smaller, but you can expect less variance, if your able to play many tables, your win rate should be close to playing one $6 table. The rake is worse at these lower limits, but so is the competition. Also another advantage is that you can play more games and get more experience.