I like the Ego-Twist batteries, but if you don't care about the sleekness of your unit, you might find yourself well-served by a slightly larger model, like a "avatube clone type. If you shop around, someone always has a sale on 'em for about $50. You can replace the batteries easily, they're rechargeable, and they take any standard 510 atty, carto,
tank, or similar, which gives you future expansion options. I presently use a lavatube clone with a Phiniac XL tank and punched cartos, and I fill the tank and swap the battery about once a day, which is nice. If you want multiple flavors, you can prefill some cartos or clearos and swap out to your heart's content... but after a year into this, I've found a lavatube clone with tank to be a stable, reliable, surprisingly non-finicky option, if larger than an ego type.