2820 or Zumo
If you want to use your computer to set up routes in advance to go on road that YOU want to ride not the roads that the satnav chooses (eg when planning a foreign trip I would go with the 2820. )
As far as I'm aware, you can do this with the Zumo but the unit will still recalculate the routes when you upload them meaning you may end up on some roads you didn't want to go on. (unless you set loads of waypoints which can be a pain - Much quicker & easir to plan/review the routes on a PC.)
On the 2+++ series you can disable the 'off route recalculation' so that you always go where you want to, not where the unit wants to take you. If you do go off route, you get an arrow directing you back towards it rather than a total recalculation. Main reason I've never upgraded my Navigator II (2610)
(Unless they've now altered the zumo ,,,anyone ??)