I own a DSE 901, RN 4075, and Joye 510, so this is all speaking from personal experience, as well as several months of reading on the forums.
I know the 510 is the popular choice, and it is what I would recommend as the next level after your first good eCig experience.
But many users have difficulties with the 510 for various reasons. There's a learning curve associated with it.
So I recommend a DSE 901 or an RN 4075 for a complete beginner, because they are more consistent in their behavior and have less of a learning curve. The 901 is adequate for a first experience if you use a good liquid with a high enough mg (if you are using nicotine liquid to imitate the analog experience). The RN 4075 gives a hit just a little shy of the 510's.
The 510 is the popular choice for a portable PV for experienced vapers, but look at the number of threads of new users having problems with it and becoming frustrated by the experience. It frustrated me so much I set it aside for months after never getting decent performance out of it and used my 901 exclusively until discovering the 4075. It was only after starting a problem solving thread on the forums and working through the issues with experienced users that I was able to enjoy the 510, and even then, I am still having issues of various kinds like cart leaking that I hope more practice and experience will help me to resolve.
That being said, I would recommend researching the forums and looking at vaper's comments about their various two-piece models, three-piece models, pen styles, and oversized mods like the prodigy, bullet, protege, etc. The more research you do before making a decision, the better.
And when you do buy, look around for the best price, and check in the supplier's forums for the coupon code list.
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