I would say that Brother, Janome, (who also make John Lewis own brand) Singer are all good machines. I have used and taught on quite a few different brands and the only one I think I would avoid is Silver. I would also not recommend those darling little cute ones that sell for around £50, they just don't do enough.
Mechanical machines are:
less expensive
can be repaired
easily maintained.
Some have a lot more stitches that others, ask yourself how many you will realistically use.
Computerised machines are:
more expensive but not as much as they used to be
As expensive to repair as to get a new one.
Are easier to use as they have speed control (good for teaching children)
Always stop with the needle down.
Don't need the thread to be pulled to the back when starting a seam, so less likely to snarl up.
Can have automatic thread cutters.
If there is a shop near you, as others have suggested, it is good to play around with different ones.
A word of warning about computerised machines. Once you have got used to one you will never want to go back to mechanical!