I don't have a Dakota, I have a Montana. It's comparable in functionality (albeit with more memory, more geocaching capacity etc) and is very significantly bigger than the Dakota.

A lot will depend on what you're wanting the GPS to do. If you want something bike-specific you might be better off looking at the Edge series. If you want something for walking, hiking, geocaching etc then if you've got the budget you might find a physically larger unit easier to work with. That would probably mean an Oregon (go for an x50 series rather than an x00) or the Montana.

The Montana is a big brute of a unit and there's simply no way to attach it to a bike if you're worried about aerodynamics. That said, those of us in this forum are unlikely to be the most aerodynamic riders so that may not concern you (it doesn't concern me!)

My friend has the Dakota 10 which works much like the 20 except it doesn't have the memory card or electronic compass. There may be a couple of other differences as well.

If you like geocaching then I believe the Oregon has a Wherigo player built in whereas the Montana does not and I'm pretty sure the Dakota doesn't either.