Indeed Hrun is right... sadly there isn't (that I know of) a watch on the market that can continuously measure heart rate without a chest strap... there are watches available with metal contacts on, which you can hold your finger on and it will measure your HR, though obviously this would be pretty awkward to hold your finger(s) on it whilst running along!
The Garmin does look quite big in pictures, but for function it's fantastic, big easily readable display and not too weighty at all.
Footpods
can be very accurate, there are different types - those like the Nike+ system, which is basically a glorified pedometer and works out distance based on something along the lines of AVERAGE STRIDE LENGTH x NUMBER OF STRIDES = TOTAL DISTANCE. There are more complex units that have a bit more science behind them and are more accurate at varying paces, I think they have sensors in them that detect the force/speed/acceleration or something of the foot. Though I'll be completely honest and say I have no experience of this type; only the Nike+ type which really didn't suit me (my stride length varies a lot from races to recovery workouts, so readouts for me could easily be 1/2mile off over a 4mile run)
If you're concerned about the look of the watch and it's size... the Garmin Forerunner 405 is a much sleeker and more 'normal' looking unit, though for HR it will still need a chest strap.
I can't see any problem in wearing your chest strap underneath your underarmour, in fact it'll probably make it even more comfortable! Though of course if you're worried about appearance and don't wear anything on top of the underarmour then yes, you'll probably have to put up with a weird bulge that looks a bit like a bra strap from behind
