To Garmin 310xt or not to Garmin?

Hello to all. I am a beginner to tris and am building my gear as I build on my training and knowledge. As such I've taken the approach of trying to invest wisely on gear and in time. I am looking for advice on the purchase of a Garmin 310xt. My goal is the Madison IM next year, with sprints thru Oly and possibly a 1/2 this year if timing and training time allow.I have an iPhone, a base model Polar HR monitor and a Cateye Strada on the bike. With the iPhone, I can map and track my routes. The Polar works thru the swimming (albeit questionably) and the bike computer logs all the typical info less cadence. What intrigues me about the Garmin is it is all integrated into the watch and it's on sale at Nasbhar thru noon today for a good price. After adding the footpod and the cadence sensor I'm looking at close to $500. I'm willing to make the investment if wise, but not frivolously.I could grab a bike computer to cover the cadence gap I now have, and the HR monitor works fine. I would like to have some pace monitoring capability, which the iPhone can do, but clumsily. No to mention it's battery endurance wouldn't last thru much more than a 1/2. My bike is setup as I need and I also have race wheels.What are your opinions? Is this THE training tool that can make a significant difference in obtaining my IM goal provided all else (time/miles) is invested? TIA