As Boggy rightly says, the extra resolution does make a difference on the 556 and if space is tight there is no better way of getting a large range of data on screen. The 620 is a delightfully compact option too.
If space is less of an issue have a look at the
750 - really stunning value for money!
Unless you want broadband radar and downscan sonar, I don't think there is a multi function chatrtplotter that can compete with Garmin at the moment.