I work for Sprint and yes, Mobile Broadband over 3G or LTE from a carrier is the most secure. It is totally encrypted between your carrier provided modem and the Internet. If you use a Mobile Broadband WiFi hot spot, be sure to use the strongest encryption mode it offers, usually WPA2 is the strongest unless your computer can't support then use WPA, WEP is pretty weak don't use it if you have a choice. Pick a LONG password for it (at least 9 characters) with lower case, upper case, and numbers. Hacker tools can try all the short passwords up to 8 characters long one by one and will try all the "simple" ones first. If you use a 9 character WiFi password, your chances of being compromised is near zero.
If you use public WiFi or a private WiFi with password from a campground, MickeyD, Starbucks, etc...
What web sites you are using can be monitored. Your in/out email can be monitored, UNLESS:
If you use the web and see a web link that says https at the beginning of it, your data in both directions are being encrypted and even if others intercept the packets on the wifi they won't be able to use them for anything, other than being able to tell what web sites you are surfing to. Most anything related to credit cards, banking, ordering on line uses SSL, indicated by the https in the URL. Now, even things like google are able to be used with SSL use so that people can't see your searches and search results. Try typing https:// in front of the site you are using, for example:
https://google.com, often it will still work in the encrypted mode.
If a hot spot has a password that means that only those that have the password can monitor your packets. In other words, still not very secure. Unless the campground changes the password very often. Anyone with a laptop and some hacker tools in range of the WiFi can sniff your packets, but the ones with SSL use powerful encryption that takes super computers to break, like what the US government has.
For your emails, you can also set your email client to use SSL (secure socket layer) for both send and receive and then your email will also be encrypted as well. Check with your provider for how to make your mail encrypted, it varies by provider.