I have the bike cadence/speed sensor for my 305 (I believe it's the same unit that all Garmin units use for the bike) and it makes it the most awesome multi-sport device ever...I use it on my runs, as well as for a bike computer. It's perfect...and it works great on the indoor trainer, since the cadence sensor works with the back wheel--so I have been able to track my trainer miles all Winter.
I don't have the foot pod, but I'm not too worried about my cadence while running, since that's done outside about 100% of the time.
Someone I talked to said that they thought the 301XT felt LESS bulky than the 305, even though it looks a bit bigger.




. It looks even bigger than your 305 but I don't know if I care it's not like its going to be my new fashion statement or anything. Historically I have bought HRM based on how they look as watches, I think I'm past that now. 
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, but it's easily replaced with a retaining strap like the 301 used to have.

