Ok, I'll take a stab at this.
I've got 401s and 901s, have not tried a 510.
401 atoms - 3 types I've run across - all take cigarette-diameter round carts so mouth feel is similar to an analog; lighter in mouthfeel than a 901.
type a) the dinosaur lungpullers - 2hole - narrow bridge or pointy bridge, opaque plastic gasket thingy completely around the base of the bridge up to the base of the bridge - 4 of 5 pull a lung up on a battery -better on a PT - 1 will only pull on a PT - of the 4 that work on batteries, they seem to work better with particular batteries. tossed in the drawer for armageddon stash.
type b) cromagnons - good draw - 2hole - wider, rounder bridge than dinosaurs, metal under the bridge, clear plastic gasket thingy around the base radius but doesn't extend in up to the bridge as on dinosaurs - also persnickety about which batt they are on - all work just fine on a PT, so mine are now permanently assigned to PT duty and armageddon stash.
type c) NextGen -4hole - the stars -best draw (harder than a 901roundtip, similar to a 901whistletip) - wide bridge, white plastic under the bridge and around the base radius, no gasket around the base. Some work better on particular batteries but all will work with all batteries - all are stars on the PT - these are my everyday all day atoms. Definitely a few in the armageddon stash.
Cleaning - easy - exposed atom makes blowing it out into a papertowel very easy, and battery contact is recessed below the threads which allows for complete cleaning of the internal threads right down to the innermost one.
Carts: - a pain - Cart inserts are teeny and a pain. If you have arthritis or are far-sighted, you will hate them. Straw mod works well. EVO cart inserts are much larger (oval with a tree-trunk leg) and easier to handle. Both inserts must be pushed up with a paperclip to get them out, then pushed back down with a paperclip and, if not an Evo insert, pushed back up 1/8th an inch with a paperclip to allow airflow and seat against the bridge properly.
Positives: most analog-like in handfeel and mouthfeel as well as appearance. Carts have a silicon stopper so easy to change carts and go. Withstand abuse. Don't mind the thicker, VG-based juice. Smaller carts help remind you what your intake is. Never had a 401 leak.
Drawbacks - Bridge is not protected if you drop it and can 'dent'. Carts are tiny - about 1 to 3 cigs worth - on a good day. small parts, so harder to handle, must dip instead of drip (but you can turn sideways and quasi-drip), seems easier to flood than a 901. 401 atoms seem to need a 'break-in' period, less for the 4-hole than then 2-hole, but generally 2-3 days before optimal vapor.
Compatible with: nothing (m401 aka EVO aka Pilot compatible with m402/403 only)
901 - I have two types - SLB from E-CS and unmarked from MFS - both styles have a raised bridge in a long tube - one is pointy (MFS), one is more rounded. both have metal up to the base of the raised bridge. Can't tell much difference in draw between pointy bridge and rounded bridge - maybe the SLB is slightly easier. Both comparable on a PT.
Cleaning: Atom itself is harder to clean than a 401. 901 has recessed bridge about an inch below top of tube, so harder to clean/dry interior of tube. 901 also has an extended centerpost on the thread side of the atom, so, again, harder to reach the innermost threads for cleaning.
901 carts: pull off the mouthpiece, and there's the filler, no paperclips required. Accept straw mods easier than 401's (more room for them).
Two types of carts available:
a) Round tip cart is much easier draw than a 401, and delivers slightly more flavor, but not by much. The round tip cart is slightly fatter diameter than a 401. A little Harder to hold between teeth since it's heavier.
b)The whistle-tip is a pinched together mouthpiece. similar draw to the 401, not as easy as the round tip. I personally do not care for whistle-tips.
Positives: cart holds 3x the liquid of a 401. Bridge is protected if you drop it. One piece, no tiny inserts. Easier to top off. Can drip. Didn't seem to need a break-in period. Large carts make it easy to marathon vape.
Drawbacks: harder to clean- 'condom' on end of fresh cart gets wet with juice so is slightly messy to remove before use (moreso than the 401 stopper). Round-tip collects juice inside the tip and needs to be blown out - no way to directly dry the inside of it off. Need lighting to see inside the tube to drip on the bridge. (is there another way I'm missing?) 901 seems to burn hotter than the 401. Not as conscious of how much juice you're really vaping.
compatible with: many mods use the 901 atomizer. I've read 901 carts can be used on a 510.
Vapor production: on both, using VG-based liquid, I get pretty equal vapor production. 401's needed a few days to break-in and mellow before optimal vapor. 901's vapor dropped off as battery weakened, 401's stayed right up there until just before the battery started blinking to recharge. Both show increased vapor on a PT. Can't speak for PG only vapor, because I cut everything down with VG if it isn't VG already.
Flavor: Seriously can't tell the difference in flavor on battery between the 401 and the 901. More flavor from both on the PT. More flavor when atom is cooler, and the 901 seems to burn hotter, faster with VG, so flavor dropped off accordingly. Then again, my tastebuds have only been off analogs for 60 some days and are still waking up.
Throat hit: what's that??? cause I haven't felt one (maybe because I use vg?)
Personal Preferences - Age-based = mature adult (that's a PC way of saying old as dirt)
a) at home, out and about driving, in public, at office, wherever I need to appear 'professional', when I want to count carts and limit intake, when I want a cigarette-looking device = 401
b) at the computer, 901 round cart (have I mentioned I don't like the whistletips at all? lol).
Overall winner IN MY OPINION for my agegroup = 401, 4hole atom
YMMV
Hope this helps!
