I use a Sigelei 150W and the Subtank Mini for when I'm out and about or if I'm not in a situation suitable for dripping. The full size Subtank will be slightly too big for the Sigelei, at 25mm and so you'll get a bit of overhang (the mini and Atlantis are both 22mm).
My Atlantis felt great and this is coming from a previously strictly dripper and RBA guy. However, the mini blew it away. To me personally, the Subtank Mini as at least marginally better than the Atlantis, although I'd say that it's miles better. Many other reviews seem to share the consensus that the Subtank Mini is better than the Atlantis.
Now. The Atlantis holds 2ml and the ST Mini holds 4.5ml. They are both the same diameter (the full Subtank holds about 6ml I think. The full subtank had a few issues which were resolved in the later Subtank Mini.
All in all, I say get the ST Mini along with the 150W Sigelei. The Kanger OCC heads are wonderful and the included RBA section is surprisingly good. The OCC heads are also exceptionally easy to rebuild compared to the Atlantis heads, so you can save yourself some money on the heads.
Added:
The Subtank full size is 25mm and the Sigelei is 23mm, so 2mm overhang. Barely noticeable but I still would have said to go for the mini - more capacity than your other option (Atlantis) anyway, will sit flush with the Sigelei, and is simply improved and modified based on the mistakes made with the original Subtank