I've had a Garminn i2 I think it was (the cheap one) and now have the TomTomOne. I think the Garmin was better value for money. I didn't have all the bells and whistles of the TomTom, but you could add POIs and Speed cameras. It was a little bit time consuming to enter addresses with a wheel scrolling through the alphabet, but it did pretty much the TomTom does. The TomTom has extra voices and the option of adding extra maps, but if you don't want this then why pay extra for it. I also had a Navman iCN 330 which is a waste of time and space and actually cost more than the TomTom. Leave Navman well alone. It sent me into a multistorey car park. If you like gadgets then go for TomTom, it's more fun than the Garmin. TomTom is easier to use, but when I bought mine, about 70% more expensive. If you shop around online, you may find the earlier version of the TomTom which has a better windscreen holder and a different colour box. The software is older but can be updated when you connect to a PC connected to the internet. It's really easy. Even my old man managed it. Garmin doesn't take map updates (The i2 didn't anyway and neither are software updates available or voices which are great fun!)
The modern Garmins take updates, but maps are very expensive so you are probably better off getting a model with european mapping if you are going to the continent.
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Having used both Garmin and Tom tom I personally prefer the Tom tom, there are plenty of POIs avaliable for both, the Tom tom one Europe has much more up to date maps, even for uk use than the more expensive Tom tom versions that use a different mapping, so my choice would be Tom tom one version 2 Europe, but Im sure what ever you get will be of great advantage.