Smartphones work great for creating traces of where you’ve ridden (nice for backtracking) but are generally not as accurate as a dedicated device.
The GPS system is set for a big upgrade in 2014 when it will have access to a much stronger signal. It should be 1-meter accurate with quicker lock on times vs the 5-meter accuracy of current devices.
If I were you, I’d probably go for the smartphone now to get started, then buy a nice unit that can make use of the signal. Or just buy a cheap dedicated unit now (~200).