Garmen 310 or 405 ?

There is no swim function with the 310, it is just waterproof so you can wear it the whole time.  It will record your path in the water but from what I have seen that is spotty.

It is also smaller than the 305 and supposedly has a much longer battery life.

Really the 310 just adds some features that the 305 is missing.  I like the fact you can swim with it and don't have to worry about trying to put it on in T1.

The 310 is ANT+ compatible so if the Power Meter you have will broadcast ANT+ the 310 can pick it up (haven't heard of any ANT+ PM's not working with it.).

No idea about the 405 though.

The 405 is a decent GPS for running, not so much for the bike, and it has not multisport mode, it's got a much shorter battery life, and as soon as you start sweating you can't really use the bezel to change anything. The 405 & 405cx differences are I think the CX has HR based calorie consumption. If you want one for triathlon's get the 310, and if you don't want to spend that much, get the 305.

x2 on the 305. Nearly all the features of the 310 at half the price.

Have all three of the watches you mentioned, the 305, 405 and the 310. I loved the 305 for running and I have logged a ton of miles on it. bought the 405 for my GF and to be honest I am not a big fan of that watch. The bevel thing does not work very well and the previous post about limited functionality while sweating if 100% spot on.The 310 is by far my favorite-even though I have only used now for a few weeks. It has some features I wished the 305 had. The 310 is more than a waterproof 305. Take a look at it, it might fit what you need. Cant go wrong with the 305 or the 310.Good luck!

Such as?

Waterproof
vibrate alarms
20 hr bat life
update GPS firmware
more options for data screen info
power meter compatible
new heart rate monitor stap.sensor
new smaller footpod
ant+ data transfer 

Yeah, all that's OK, but most represent only marginal improvements over the 305. Uuntil I get a powertap it's not worth the extra $150 or so.

I can see your point....but for me the cash was well worth it.  Who knows what Polar will come up will to "counter" the new garmins.....

I see some of those as major improvements, absolutely worth the extra $150 as far as I am concerned, if those improvements are not a big deal for you, stick with your 305 (which it sounds like you have already decided not to do). Back to your original question, I would go with the 310 over the 405, I demo'ed the 405 before I purchased my 310 and liked the size...that was about it, for running and triathlon and when it comes to having a device that has utility in triathlon races, the 310 is great.