Depends upon what you want and what cartomizers you may already use. Do you already have a favorite cartomizer?
Both length of tanks and cartomizers work equally well. If liquid capacity is a factor, the XL length will have a larger capacity than the same diameter standard length tank.
The pre-punched Boge cartos are more easily to be found in a standard length, although there are pre-punched XL Boges available. Smoktech is much more likely to be found in the XL, or tank replacement, size. (I don't even know if Smoktech makes a standard size carto.)
Although I don't recommend them, dual coil carto's can only be found in the XL size and only made by Smoktech, to my knowledge.
There are differences in the two major brands of cartomizers, that being Boge and Smoktech:
Boges typically have a more dense filler. This can make them slower to wick when filling, slower to wick in a tank and possibly lead to dry hits or burnt taste, but less likely to flood. If you find you have frequent issues with your carto having burnt taste or poor wicking, perhaps give the Smoktech's a trial.
Smoktech's are just the opposite. Less dense filler making them faster to wick when filling, but easier to flood. If you find that you have frequent issues with flooding, perhaps give Boges a trial.
The above are my personal observations and other's may agree to disagree with me.
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I recommend that you review the blog articles that I have written, which can be accessed by clicking on the link under my avatar. They include the following topics:
1. Descriptions, pics, and videos of the more common
juice delivery devices.
1. List of vendors that sell pre-punched cartomizers, the safe
vaping power chart (how to choose the correct resistance for your voltage range), editorial on why I prefer laser-drilled cartos over self-punched cartos.
3. Detailed review of the IBTanked pyrex cartotanks.
4. Beginning vapor's first starting setup.
5. Cartotank setup recommendations, why pre-punched carto's for newbies, single coil vs dual coil cartomizers, a demo/review video of a popular tank tool.
6. Juice delivery devices safe to use with cinnamon and citrus flavors.