I have owned a S720i now for about 6 months and have found it to be really usefull. the fact that i can look back at the ride and compare the height, temp, Hb and speed are a good way of seeing improvments.
as for the extra features offered by the 725, i cant for the life of me see why i would want to check the results on my phone.
Yes the chest band is an updated version, but the one i have it quite comfortable.
Now there is a newer model you can pick up a cheaper 720 rather then a 725