All public wifi are running at "G" (56 Mbps) at the fastest, some may run a little faster but not many. Public wifi are also setup with security blocking downloads of large sizes or heavy usage.
The 3G will be average for public in a lot of places but that's not the same as a wifi that your connecting to at a coffee house for instance. 3G is cellular service and iPad uses that but with a cellular plan you get thru your cell carrier. WiFi is from a business like a coffee shop that they setup for customers to use while in their restaurant. Usually that's just a guest network which is at the max, a "G", you won't find any public wifi running at "N" speeds.
If you are having problems connecting to a "wifi", the iPad may be connected to another wifi connection already. The iPad also would need to have the data turned off on the iPad, you can't use wifi if it's connected to your data package. Once you turn off the data, you should pick the wifi you want to use without issues. Then once your done with the wifi, then turn the data back on then you will be back on your data plan.