Update June 2019
The “Mobile arena” has changed quite a lot since this was first written and some of the boundaries have been fudged and “always on” connections are now more the norm, where your phone is connected to the Internet either by “Mobile data” or WiFi.
This bit is about Internet connections using the mobile phone companies network. In the UK, the mobile phone companies still give precedence to voice (phone calls) and DATA tends to be “slotted in” where possible (this is probably not the case if using 4G but voice and text are still separate channels to data). I think it is fair to say that although “Mobile Internet” has been used successfully for years using older technologies like EDGE and GPRS, it didn’t become popular until the advent of “3G” (Third Generation) and even more so with “HSDPA”. 3G and the newer technologies (4G and now 5G) give the capability of having an “always on” broadband connection which can be just as good as some (wired) ADSL/Cable connections. The main problem is that you are governed by the signal, just like a mobile phone. If your signal is poor, the “experience” will be poor. Like WiFi, this type of connection is basically a “Radio” signal and is subject to the same constraints, e.g. Interference from a variety of sources but generally when you have a good signal it works brilliantly.
From a security point of view, it could be argued that this type of connection is more secure than WiFi but it is even easier to convert your laptop (or PC) into a router so this in my opinion makes it less secure. This is probably more of a concern to company users than private households. However, as with WiFi, switch off the Mobile Connection when you are using Ethernet (wired connection).
One of the brilliant changes in using “Mobile Data” (if your Laptop/Tablet doesn’t have a SIM card facility) is the use of “Tethering”; where your phone is used as a “Mobile WiFi Hotspot” and you connect your laptop/Tablet to the hotspot. The Internet connection then uses WiFi from your Laptop/Tablet and then uses the “Mobile Data” and this is fabulous but WiFi, “Mobile Data” and Bluetooth are all “open to the air” radio signals and can be compromised. This risk can be reduced by using a VPN or some other form of “Encrypted transmission”.
Avoid connecting to banking and other sensitive type sites unless you use a VPN.
I suspect that “Mobile Data” will become the “defacto” standard because we all tend to use “Smart phones” these days and the use of Tablets/Phablets (combined tablets with phone capabilities) is probably gaining more in popularity than Laptops and Desktops. “Mobile Computers” will start to replace laptops/Tablets/Desktops to give the “always connected” situation. We already have the technology and kit to enable us to use our “mobile computers” out and about using “mobile data” but when back in the office or home, the device connects to a big screen (with keyboard and mouse.) It will be possible for the device to use whatever connection it can find, so when out and about, it will use “mobile data” but when back in the office or home, it will use WiFi. If there is a “Dock” facility, it could then use Ethernet. It won’t be easy to determine what connection your device is using but the most secure and reliable will be Ethernet. Unfortunately Ethernet usually involves physically plugging in a cable, unless you have a Dock.
Exciting times but you need to remember that anytime that “Mobile Data” or WiFi is used, these are “free to air” radio signals and the data transmitted can be compromised unless it is encrypted or you use a VPN
Use the basic security advice.