I reviewed the 610 on some post... Funny, I can't remember where
If you are looking for a GPS watch with a countdown timer and still maintains other functionality like alarming at splits (i.e. every time you pass a mile) then the 610 is for you.
Pros
Smaller than the 300 series.
More functionality with additional data reporting variables available.
Better sat reception - leading to quicker on
Vibrating alarms in addition to the sounds.
Vibrating alarm prior to going into sleep mode (the 405 will go to sleep and if in a starting coral, you may not hear the unit beeping - only to find the unit in power save mode when you hit the timer button)
Watch function. - I wear the watch when on the road so I do not have to worry about resetting for different time zones.
Magnetic charger - no plug to wear or rust out like the 300 series and the charger is magnetic so the watch stays on the charger cradle )the 405 has a tendency to come off the charger if jarred)
Its touch screen so no buttons or bezels to worry about while running.
Glove capable - you can rotate through screens on the run with a gloved hand
Cons
A loss of pixels when viewing in quadrant read out (4 squares) from the 305. Though more than the 400. More screens so you can get most everything you want in a double screen mode
The back light does not come on at the alarming of an event. I miss that from the 305
The unit occasionally hangs up when dumping data to the computer - draining the battery
Sometimes when scrolling in edit mode, I do not have enough finger nail to catch the page to scroll - the result is that you open the field you happen to touch rather than scrolling
I would say cost but they are now discounting the 610