Get the 2.4-2.6ohm ones. It doesn't affect the battery so long as you don't go below 2.4ohms on the small battery. If you figure that they will be an average of 2.4 and the si cartos are 2.7, you will lose less than minute of battery life on a charge.
The difference in the performance is huge though. Since the coil is on the bottom(like a si carto), the vapor cools down a little going from the coil to your mouth. The lower resistance one will give you a warmer
vape and makes up for the fact that the cartomizer is longer, especially if you use a drip tip.