Generally, there's very little difference. Some boxes might have slightly better 'sensitivity' (i.e. the ability to 'lock onto' weak signals) than others but this won't matter if you've got a decent aerial. (There's also no evidence to show that paying more money buys a box with better sensitivity).
However, there are a few features which you might find useful and are only found with some boxes:
1. RF Modulator: This is a piece of circuitry which enables you to connect the box to a TV via the TV's aerial socket (rather than via a Scart lead). It's essential if you've got a TV with no Scart socket. Otherwise, you don't need it.
2. Twin Tuners: If you've just got a 'standard' Freeview box, with a TV and a VCR (or DVD recorder), you can't watch one digital channel while recording another. (That's because there's only one digital tuner, inside the Freview box, in the set-up). If you buy a box with twin tuners, you can feed the signal from one to your TV, while the signal from the other tuner is sent to your recorder.
3. Timer programming: Some Freeview boxes have a built-in timer, which automatically changes channels at the time you've programmed into it. Most people never use this facility but it can be useful if you record programmes (via a VCR or DVDR), while you're out. For example, if you want to record two one hour programmes, at 1900 and 2200, you can feed the signal from the Freeview box to your recorder, and set the recorder to record 'AV' at the required times. With a simple Freview box, this would only work if the two programmes were on the same channel. However, if the Freview box was programmable, it could be set to change channels between the two programmes.
Chris