The 300 is UK only and the 500 has European maps too. The 700 has some additional phone dial features.
It is a good product and if you have never used sat nav then it will be great to use. All the equivalent products from other manufacturers work fine, but just do things slightly differently. I would not say that there are 'better' alternatives, but perhaps 'different' or just a different feature that may be better for you.
The maps should be upto mid 2004, I know some of the new roads (completed last summer) around by me are on there - but I am using TT 5 on a PDA, but it should be the same.
Check out the forums and review sections at
www.pocketgps.co.uk.
And if you do buy one, don't forget to downloada the POI (points of interest) databases from the above site - especially the speedcam databases.
A cheaper alternative may be a PDA set up - typically a Mitac or similar unit for around the £200- 230 mark. Software may not be as good but still works fine. Then install TT software if you want to.