Hi Matt,
I'm in that same dilemna as you and looking for a monitor that basically does all... I tried to go the moderate expense route but there were always things that sounded like they would be very useful later on that were available only on the higher end models.
I read many MANY reviews of the HAC4 vs the S710 and the two points that are always consistent are:
1) The HAC4 is an excellent cyclocomputer with a HRM and the Polar 710 is an excellent HRM with a cyclocomputer...
and
2) the power output (wattage) measurement of the HAC4 is useless because the results are "derived", whereas the Polar 710 power output measurement is much more accurate, but you have to spend $300 for the add-on...
What has me stymied at the moment is that there is talk that the Polar 720i has "addressed" many of the issues that made their 710 cyclocomputer so disliked (lengthening the amount of time for "stops" and display of trip distance on the display instead of only from the download etc...)
If its true (and haven't found reviews that say it is) I think the Polar unit might be the more useful piece... Wish I could have a chance to fiddle with both before purchasing...
Have a good one!
Feanor