Personally, for flavour, geekvape zeus dual wins hands down. See my other thread on the obs engine 2. Despite having watched several videos online and trying many useful suggestions from the guys on here, i just couldn't get it to perform, and it wasn't for lack of trying either.
Currently running a 3mm (or marginally larger i guess) 7 wrap single 22g ni80 flapton in the gvz at 52w and it blows the obs out of the water. I tried dual coils initially, but its a bit fiddly and also makes the already sensitive wicking even more of a challenge (see below), so ironically, a single coil large diameter is a doddle to build and wick.
Gv is definitely less restricted than the obs, almost too airy for me unless i dial it back a bit. Obs even full bore is more restricted. Is a tighter draw than my kylin mini also. Probably due to the tiny, somewhat fiddly build deck on the engiine 2, as when wicked there is little room for airflow around the coils.
My crown 3 standard prebuilt coil tank feels more free flowing than the obs too.
Obs wicking and gvz wicking both seem to have little room for error. The obs has no flavour or floods if its not perfectly done, and the gvz dry hits like a mofo if you have too much in (even though if you believe any of the reviews online, you are supposed to be putting enough wick in to poke down slightly into the juice ports), whilst too little and it spits and gurgles terribly Imo, the gvz needs wicking to just about reach the juice hole openings when dry. Prime it, and it should then it should just go into the juice holes. You also absolutely have to thin the ends out, and quite a bit too. On a 3mm coil you almost have to half the wick density at the ends, in my experience. Once right though, its banging.
Overall, the gvz is the easier and quicker to build and wick.
Ive got the bubble glass for the gvz too, and it looks nicer imo, and also holds about 6ml juice.
Honest opinion, if you like flavour, and that takes prioroty over clouds (although, the GVS can chuck out as well), with a free flowing draw, the gvz is what you want. The 510 drip tip adaptor is one of the neater solutions ive seen, and is not as fiddly tomget in and out as the reviews imply, in fact infind it easier to accidentally knock the 510 and this adaptor out, however the 510 included is actually not a bad tip. Ive tried all 3 drip tips included, and decided to stick to the standard fitted 810 as it reduces the overall height. Its not my favourite feeling tip if im honest, buut i think you get used to it after a whole, despite it feeling like youre chugging on a drainpipe initially.
Hope that helps mate.