This is very much a personal choice. The refillable tank system e-cigarettes might take a bit of getting used to, but they allow you to use more flavours, and generally deliver more nicotine than e-cigarettes that look like tobacco cigarettes. Users tend to say these types are more satisfying. Specialist e-cigarette retailers can give you advice, and you can also chat to other e-cigarette users on a range of internet forums.
How much nicotine you need will depend upon how much nicotine you’re used to getting from your cigarettes. And, of course, how much nicotine you get from your e-liquid will depend upon the type of e-cigarette that you use and how you use it. As a rough guide, most 20-a-day smokers find that 18mg/ml (1.8 per cent) nicotine is sufficient, so you could start with this and see how you get on.