310XT good for heartrate/zone training?

I have a 310XT as well as a 910XT (wife bought the 910XT for me as she thought I needed an upgrade).  I MUCH prefer the 310XT.  It isn't going to help with the swim other than to record time (unless you put it under your cap then it will also be able to record speed/distance outdoors).  You can set multiple HR zones.  It will pair with most power meters, speed and cadence sensors, HR monitors and foot pods.  One of the best things about the watch in my opinion is how well the GPS works.  I can sit in my living room, turn on the watch and it will find the satellites - INSIDE the house.  It has no trouble with trees.  Going under a freeway/highway overpass it obviously will lose the satellites however it will reacquire immediately  after you come out from under the bridge and it will "fill in" the under the bridge segment.

I have used the watch on both rolling hills and in the mountains and not had any problems, however I have not been down into steep canyons with it.  I suspect, like any other GPS device, it would not do well at the bottom of a steep canyon (that is a limitation of how GPS works rather than a limitation of the watch.

Earlier this year I traveled from So Calif to Indiana to run in a half-marathon.  We drove across the country and I would go for a short run every night.  I never had a problem with not finding satellites or being able to lock up.

Assuming you don't need the swim metrics offered by some of the newer much more expensive watches (910XT, Fenick2, Ambit2, etc) you will not be disappointed and you will save a good deal of money.  You can find good prices on Amazon and gain added value by bundling with a HR monitor.