andrea, on our last 9 week trip we had part of it with a new leased citroen and part with a rental car. rather than rent a gps with the rental car i bought a new gps, but used it at one stage alongside the in -car unit in the leased citroen. the result was my new TOM TOM worked better.
listening to two gps at that same time was quite an experience but it was a good test!!
i have used various gps over the years, and others will have their preferences. what I did like about the TOM TOM ( and did not know when I boght it) was TOM TOM developed their own 'map codes' sort of like a number system that represents an address.( like northings and eastings but easier)
so before our trip I looked up the 'map codes' of our hotels etc and then we just put them in when we were there, as required.
this doesn't sound like much but as you probably are aware the names of towns can be VERY long and often duplicated , so it can take time to sit there trying to work out if the address is correct. whereas the 'map code' is only a series of about 6 numbers , for memory. too easy!!!
in addition to a gps we use Pocket earth, absolutely positively would not go anywhere in europe without it.( we use it predominantly on a ipad mini) I download the maps at home ( where wifi is good) and then use it offline when we are there. it is so handy. think of an offline capable street directory ( with many extra features) of the 'whole world'. when your gps gets it not right ( and they all will at times) it can be a very handy backup. its also very handy on your iphone when just working the streets as you can measure distances with two fingers and see what's in the next street etc, which can save walking.
we also buy some michelan maps as an additional resource, and memento, ( when we are there , so the names are spelled correctly) but they don't show the detail that Pocket earth shows.
I hope this helps.