The operating systems seem to be much of a muchness. I've used Tom Tom on a PDA, and have owned a Via Michelin for the last 18 months. Both seem to be about 98% accurate.
As for the screen - definitely go for the largest one you can afford, it makes life so much easier. Software can (and should) be easily updated - but you're stuck with the screen.
A worthwhile site is www.pcketgpsworld.com You can subscribe intermittently (via Paypal) to their constantly updated speed camera database (which covers all types of cameras, from speed to traffic lights)
The other obvious tips are, don't leave the mount on display. If you're using a windscreen mount, clean the marks off (scrotes look for these as an indication that there might be a sat-nav inside)
I've found the best solution to be a plastic disc, made to accept the sucker base, which is attached to a flat part of the dash with double sided tape. This can be easily covered with a seemingly carelessly placed duster etc, to avoid telling the scrotes that you have a sat-nav. It's not much of a ruse - but, then, most scrotes aren't very bright and will just look for the windscreen marks