There are positives and negatives to 2s vs 4s. 2s are more durable packs and you will generally get longer life out of them. 4s packs are less hassle going from from car to charger, and generally the cheaper option. Also, due to the extra casing dual 2s packs will weight more. For inexpensive packs I would look at SMC or ProMatch. I'm currently running Reedy 4600 packs for the weight savings.