the other pda halfords sell is still a navman unit and uses the same maps as the 510 as far as i know, tom tom is good as u can get loads of stuff for it like overlays etc very easy, and also use lots of diff software on it, but if u dont really want clutter then i would say an all in 1 is better, as with a ipaq and tom tom u need a charger for it unless u wanna use the battery but then u have 3-4 hour batt time, also if u go for a wired gps reciever u got that wire, or if u go bluetooth then u have the charger for that though u can run it off a battery.
in pc world they have a ipaq 1710 with tom tom kit for 249 i think it is, but the pda isnt the best pda in the world and the tomtom software is a cut down version of the standard tom tom 3, i really did like the navman 510, it is a very neat and tidy unit and looks cool, but it was expensive and i could of got a m8 that works there to get staff discount lol my ipaq and tomtom settup is about 350 quid alltogether but i can use it to store photos on for work etc...