Seriously? Crap, I'm more noob than I thought. What's the basic difference between a ''wi-fi'' and ''3G'' tablet? Just the max speed? In Brazil we usually just use the word 3G for the web connect stick.
Hmm...I dunno if the terminology is different but here in the states the two words have different meanings.
"Wifi" means the device is capable to getting Internet service through connecting to a wireless network of some kind, be it a home wireless network, business, or a "hotspot" somewhere like at a cafe or bookstore or something. If you aren't anywhere near a wireless router of some kind (the device that transmits and receives wifi signals) then you are SoL and JwF for Internet. Connecting to the Internet via a Wifi hotpost of some sort is usually (but not always) faster and you normally don't have bandwidth restrictions whereas a lot of 3G service providers do.
"3G" or "4G", without getting too technical, refers to the signal service provided by whatever company you're using (AT&T, Verizon, etc) that will allow you to connect to the Internet anywhere they provide 3g service (which is almost anywhere they have phone service). The plus side of a 3G/4G service is that provide you're in a place where there is phone signal, you can get internet access, which is handy if you're on the go a lot. The downside is that speeds are not always up to par with what you'd get at a Wifi hotspot and most 3G/4G providers will charge you for the bandwidth you use. For example, AT&T charges me $25/mo for 2 gigs of bandwidth or something like that for my iPad (dunno the exact numbers, I just know I barely ever reach that number unless I decided to stream a lot of movies on my iPad) so if you go nuts with the media, you could end up paying for it.
You'll notice as such that Wifi only devices are cheapers than Wifi/3G or 4G capable devices and thats because while both can connect to Wifi hotspots and such, the ones with the 3G/4G capability have additional hardware that allow them to do that (or so I believe anyway. I could be horribly off on this).
If you're going to be using it for e-reading purpose, than I would say a Wifi only device will suit you find. Having that extra 3G is handy but only if you're looking to download something on the go or surf the internet while you're up and about but I dunno if the extra expense is worth it to you or not.