I've got both, and I'm also relatively new to vaporizing.
Here's my input, take if for what it's worth:
M401
I bought this one first (well, second -- after someone gave me a smoking everywhere e-cigarette for my birthday and I loved the idea, but the thing just didn't work or produce any vapor.) I bought it because the youtube videos looked good, and it was small, and black, with a blue led, and it seemed handy and sneaky.
What I didn't know:
1. You will need some time to get the hang of it.
2. Because you will need some time to get the hang of it, it's unrealistic to expect that you might open the box, connect the battery, stick on a cart, and never smoke another cigarette again. Way too much pressure, and not very nice.
3. The cartridges are not equal to a pack of cigarettes. Not even 3 or 4 cigarettes. All in all cartridges are not a good way to go. Buying liquid, and refilling your carts is a great way to begin, and have enough juice on hand to figure things out.
4. These are electronic devices and they break.
You can blow an atomizer, kill a battery, or step on it and break it. Back up parts are really important.
Not knowing any of this, I bought a 401 and some juice and clogged it up, didn't charge the batteries correctly, blew an atomizer, and freaked out emotionally because I thought I was supposed to stop smoking tobacco. My throat got sore, my face got sore, I was sucking and sucking as hard as I could on my little 401 carts, chewing up the ends, and trying to make it work like a cigarette. I had NO idea that the trick is to use less liquid than overflow, and to pull on it gently in order to fill your mouth with lovely vapor.
Once I got the hang of it:
Now that I have replaced the atomizer, and the carts I find the M401 really does produce a nice amount of vapor. It' not a monster but it's nice. I pull on the cart softly and feel the atomizer warming up between my lips, and can fill my mouth with a nice, very flavorful drag.
The battery life is fair, several hours (3-4) and charge time is quick (1-2 hours)
It is indeed small, covert, and easy to travel with.
The best thing about the M401 is that somehow it delivers a lot of lovely flavor. Far more than the 901 and the 510. I like the 401 as an after dinner smoke, or a late night conversation companion. I don't like to use it for long. The flavors are too strong, and my mouth feels puckered and coated, and over flavored.
510
I bought the 510 recently after breaking my 401 and trying the DSE 901. I started looking for something besides the 401 because I wanted more vapor. And bear in mind here that I've only had the 510 for three days now.
It produces a lot more vapor than the 401 or the 901. It's a vapor monster. what it lacks in flavor, it delivers in warm yummy vapor.
Already I can see that the down side is the short battery life. A couple fully charged batteries, out in the wild, and three or four times already (in three days) I've found myself in a bit of a pinch because both batteries were dead, and I was far from home.
Ways around this:
Buy a charging case. Totally wicked sell one, lots of people sell them. They charge up your batteries while you're on the go. I love this and am looking into buying one as soon as I find one without a logo on it. I want plain black or plain metal.
Buy longer batteries. I have already ordered two of the mega batteries from Totally Wicked. They claim double the battery life... even if it's just another hour I'd be very happy.
Thoughts:
I think that it's pretty natural to explore, and to wonder, and to try a few different things before you find the perfect device for you.
I know that my thoughts and experience may be completely different from yours or anyone else. So no one can really tell you "Dude, the 401 is perfect, get that!" without knowing you really well. Smoking is a really personal thing. And vaporizing is somehow even more personal, and also a very different thing to do (as similar as it is, there's a lot to learn.)
So go slow. Be willing to try. Don't think you have to find the ultimate unit and then stick with it, or go back to smoking if you don't like it.
I recommend:
If you want flavor, some vapor, and a stealthy little unit, then the 401 is for you.
If you want vapor, and I mean steam streaming from your lips... then the 510 is for you.
I have both. I'm keeping both. It's just what to do with all these 901 batteries, atomizers, chargers, and boxes that I've gotta figure out now.
Hope anything I said helped you.