I change around from mod to mod like I have a spinner, 2egos, and a MVP itaste. I'm trying to decide what ohms to buy for dripping . What's the difference between 2.0 and 2.5 ohm for example...thanks for your insight and help
Given that most of the devices you've told us about are variable...either will be good
A bigger decision will be whether to go for something with a bridge in it or not.
What is a bridge?
A little plate with some mesh wrapped around it, and maybe some extra wicking inside.
Pros of having a bridge...
It'll hold several drops of liquid and let you vape longer before dripping again.
You can pop on a cartridge that can hold a few more drops of liquid (results may vary...usually more trouble than it's worth).
Cons of having a bridge...
Atty might gum up faster...
A little extra patience required to prime it and maintain the flow/wicking.
Not as seamless of a transition when swapping flavors.
More difficult to inspect/clean
Pros...an atty with no bridge...
You can see right to the coils.
Easy to prime and start using.
Simple to clean and start on a new flavor (just shake it a bit and move on).
Cons on bridgeless...
Only holds a few drops
Maybe easier to flood...
Possible sputtering of juice into mouth (a good drip tip can fix that).
Carts not possible.
Next is what platform you want...........
306 types are pretty cool in that when you pull off the drip tip...the atty is more exposed.
The catch is..they're pretty small and don't hold much liquid.
They can be a bit picky on the drip tip...you'll need a special 306 drip tip...
Slightly more prone to leaks and spills when taking them apart...dropping...etc.
Regular 510
These give you a nice wall around the atty. They'll hold a little more liquid in a go. Otherwise it's usually a very similar atty to the 306.
901
These have a more airy quality to them (easy loose draw). The flavor comes through due to the way the air hole is set up.
You'll need an 510>901/808 adaptor to use them.
They don't like but a few drops of liquid at a time...and can leak right out the air hole.
A good drip shield makes these an interesting and leak free vape....worth the trouble? Sometimes

The flavor is very good, and if you like a looser draw type, they're hard to beat.
Any 510 or 901 type drip tip fits.
801
These things are really deep, have oversized coils, and can hold a lot of liquid without spitting/sputtering/flooding.
They'll need an 801 drip tip...or you can just use the usually included whistle tip..or slap in a cart (these actually vape fairly nice from a cart...)
Most if not all will have a bridge inside. (You can rip the bridge out...but in the case of 801, I think the bridge makes it better)
Due to the depth and large diameter...they can make a really delightful and fluffy vapor, but despite having a fairly tight draw, it also tends to be a cooler and far more airy sensation than the smaller platforms (510, 306, 901).
You'll need a 510>801 adapter
If you can still find them...the old Joytech 'mega' 510 atties designed for the EGO were nice (but kind of expensive). Their extra diameter and deep well gave a vape kind of similar to an 801 or LEO atty...but with 510 threads and the ego type beauty lip.
Out of all the atties I've tried over the years...the only ones that actually worked well with a 'cartridge' right out of the box were the 801 type, and the large/untapered mega 510.
Custom Hybrids....
It's possible to find hybrid stuff out there that mixes and matches the best and worst of the platforms.
I.E. An 801 atty with 510 threads.
I've heard of these as the 357 before. In short...you design your own and have it built.
http://www.avidvaper.com/hh-357-cisco-spec-hybrid-atomizers/
Sometimes you can find various configurations in stock somewhere.
http://ikenvape.com/Atomizers/
I'll see if I can add in some hyper links with the chatter above, but meanwhile...I hope this helps a bit.