3G vs wifi is hard question indeed. There are some points for and against it:
Wifi +:
*** Wifi is more universal, it will work world-wide everywhere where you find a signal. It's standard.
*** Wifi is cheaper, initial cost *and* usage. Unless you have to buy your wifi account (which would be quite strange for mobile device like this) , you can usually download how much you want, any time.
Many universities offer free wifi connectivity for their students, there are internet cafés (OK you usually have to buy something there first), etc.
*** You can have your own, fast, completely free network at home.
Wifi -:
*** Wifi spots have much shorter range and much lower coverage then 3G.
*** Each spot can have it's own setting, password, limits, etc, which is harder for user who don't know much about it (and sometimes even for those who know
).
3G +:
*** *Much* greater area coverage - mobile operators are trying to have good coverage, practicaly 100% in cities, and quite good even in country.
*** Once set up, it will work as long as you stay in area covered by your operator. No extra settings for each spot you want to join.
3G -:
*** There are several 3G technologies, one operator use technology A, another B, C, D... and it is changing. I don't know if 3G module will work with each of them, and I doubt that it will work with new ones. I don't know these technologies enough, but I think you can find out your eReader's 3G module don't work where you live. --- Correct me if I am wrong, I don't use 3G myself.
*** 3G is more expensive, not only you pay more for module, you also need an account with mobile operator, so you usually pay fee for data, fixed payment (month fee etc), or both.
*** I seriously doubt you can use your 3G account set up in, for example, England, when you are in - for example - France. I don't know how it works in USA, it's probably much better, but in Europe, it's a problem. What if you want to read your newspapers when you are abroad? With wifi, you "only" need hotspot, which you will find in plenty in many cities.
Comments? Yes I want your comments, of course.