I think actually the 703 might be the old model and as a result of cost cutting the 702 was created and since it was different enough they gave it a new number. The OS also changed to VxWorks from Linux. Similar to how the old WRT54G went. Looking at PCBs they are essentially identical too - just the RAM and flash was reduced and the USB host port was removed.

I'm not overly experienced about the history - but had been looking at them recently.

Also I see there's a TL-WR802N which is 300MBit Wireless N in the same form factor.