In general, the larger the battery, the more capable it is for sustaining a higher voltage for longer period of time. The Ego style pen batteries are designed to be small, and lightweight. This comes at a price of not being able to support a higher voltage/wattage for a very long time. For that reason, high ohm atomizers are used on those pens.. which use less power and provide less quantity and heat of vapor. It's normal for those devices not to support lower than a 1.2 ohm coil.
There are some size standards in Lithium Ion batteries. The most common is what's called an 18650. The Istick has something similar inside of it. Those are larger in size than the pen style batteries, Because of that, they can provide more power. Most of them can support a lower ohm coils, which not only use more power, but provide more vapor. Most of them also offer a user interface that shows battery life, ohms of your coil, and power settings.
Whether or not a newer battery will last longer depends on how you vape. An 900 MaH ego using a 2.0 ohm coil uses very little power, and will last you a good long while. Not everyone needs heavy vapor all the time to satisfy them. If you were to use an Istick with your current atomizers, it would last quite a bit longer compared to your ego pen. However, if you decide to up the vapor quantity by going with a lower ohm coil, the battery time will go back down.
Now, one last note about batteries... Lithium ion batteries are subjected to a lot of sales hype. When someone is selling their battery to a manufacturer, they may list it as a 900MaH battery. That rating is only under certain favorable circumstances. under "normal" circumstances, it may run closer to 600MaH. That is common. The Istick 30 has a 2200MaH battery, but the "every day use" has been reported to be around the 2000MaH rating.
In conclusion, I do strongly suggest getting an Istick. They will more than likely last much longer than what you currently use. They have been proven reliable and are overall more durable than your standard ego pen. Because they're shorter and thicker, they are stronger from everyday stress put on them from putting them in your pocket. Think of a pencil in your pocket compared to small wooden block.. the pencil will more than likely break if you sit down, the block will just withstand the bend.