I've used both and they're both great, each with pros and cons.
Billow v2 pros: holds more eliquid, easier to build on with a slightly larger build deck and chimney. Has lips around airflow to help reduce leaking into airflow holes. Adjustable 510 pin. Gives good vapor and flavor.
Cons: glass section is slightly larger than base at 23mm, wicking can be a challenge until you learn how to do it properly. Bottom fill so you have to remove tank section and fill through slots in the base while upside down. Wider juice tips and droppers don't fit in slots so you have to take time to fill. Taller than zephyrus. Can be tight to take apart
Zephyrus pros: easy to fill, top fill using a plug. More airflow even though billow v2 is really good. You can buy coil heads for when you don't feel like building. Easier to wick. Comes apart easily for cleaning. Multiple colored o rings and seals for customization. Shorter than billow v2. Gives really good flavor and vapor.
Cons, chimney sleeve is narrower than it looks so you have less room for coil builds (largest I can get is 26g on 2.5mm rod and that's tight. Plug can be a pain to get in and out. Can leak easier on its side if you don't wick properly. If you don't close airflow when filling, you will get a little gurgling for the first few hits.
I use both on a daily basis and you can't go wrong with either. I also exclusively use ni200 and titanium wire on temperature control mods. If I could only get one, it would probably be the zephyrus since I find the flavor slightly better, but then again, the billow seems to be more consistent for chain vaping. I plan on trying to rebuild the zephyrus heads for a single coil option, but I currently have mine packaging packed up since I'm moving.
Another option you may want to look at is the new aspire triton. Looks good, but it's a little upsetting they aren't including the rba base and you have to buy it separately
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