And, more importantly, where do you store it on training rides on your tri bike?
And, more importantly, where do you store it on training rides on your tri bike?
I try NOT to use mine on rides. 
Frankly they all work so the brand/model doesn’t matter. It IS easier if yours doesn’t require threaded CO2 cartridges though since the threadless type are easier/cheaper to buy.
I have a couple of underseat bags that are fully equipped with flat-repair items and I move those bags around as I switch bikes. For racing it’s different (especially MTB where I’ve had seat bags disappear). Use electrical tape to tape the CO2 inflator AND AN EXTRA CO2 CARTRIDGE to the seatpost…that’s 100% secure, out of the way and minimalist.
The spare tube can be stored up under the seat between the rails, but needs to be secured, perhaps also with that black electrical tape.