For triathlon it's hard to argue that the 310 is the way to go. The battery life alone sells it for me. For an in-depth review of most garmin/triathlon gadgets you might want to check out www.dcrainmaker.com he has a blog that does a great job reviewing the "new stuff" and its from a triathletes perspective. Hope it helps.
Also, you can get the time on the 310xt by pressing the mode button(bottom left once) but it is small and I have to stop running to read it.
The 310XT has a watch. You just have to configure one of the windows to read "Time of Day". It's easy to do.
The 310XT also has an optional bike mount kick that I don't think you can get with the 410. This is great for bricks and races, especially when used in multisport mode which gives you the option of capturing time spent in transition as a separate time block. I really enjoy the 310XT. It's been a great training tool.
The 310XT is an excellent product! I have had one for over a year now and love it. The website is excellent for reading and tracking your training and racing times, routes Heart Rate etc. You can set it up to record your entire Tri with the push of a button after each leg. I have used many Polar and Garmin products but this one is the best I have used, at this point I am satisified and cannot find a better product to fill this need.
I had a problem with swimming with the 310 on my wrist and it getting a heartrate and sattilite reading. Someone mentioned that if you stick it under your swim cap it will lock the sattelites better. I tried it and it did give an accurate reading but it is inconvienent because you cannot hit the buttons. The heart rate reading cannot register if your in the water and there is nothing you can do about it. It isn't much of an issue as you cannot read the your HR while swimming anyway. You can get a time and your T1 out of the water and it picks up your HR in seconds out of the swim.
I have a 305 and if I had to choose between the 410 and 310 I will pick 310 as you don't have to worry while swimming with it and the battery life is great.
I put my 305 in my swim cap inside of a ziplock and works great I can swim with it and also have an accurate reading, however I don't use it during the swim leg on triathlons because there are a lot of hits and kicks all over and I'am afraid to loose it so I just leave it on my bike plus I never look at my watch while swimming.
Give it a try http://www.zonefivesoftware.com/sporttracks/
I think this is the best software for keeping track of your trainings, amaizing maps, weather info, performance predictor, etc some plugins cost a little but worth it