I'm surprised to see people swoon over a model that came out slightly more than 10 years ago..
just google in the model and look at images.
Here's a shot of it from the side coming off the wrist.
the most comparable thing I can think off is strapping a brick to your wrist. I suppose if you're a bike rider and not moving your arms.. that's fine.. but otherwise.. opt for one of the several other Garmin newer models which were completely redesigned after this model.
I have owned a 405 for about 5 years. besides the bezel issue and sweat it works great. It's been on sale numerous times in the past year or so for under $200.
The 310xt came out in 2009; you must be thinking of the 305 which came in 2003. The unit is not that large, the picture that you linked to is it being strapped onto the wrist with the quick release. If the provided strap is unpalatable, there is also the option of getting the velcro strap for the 305 (look for it on amazon).
I've had this unit for over 1/2 year after my 305 suffered sweat/water damage. The 310xt appears much better sealed, as it's really a swim/bike/run watch, as opposed to some of the smaller garmins. Read up on some of the units on the review site dcrainmaker that was linked earlier, as he has reviews on virtually all the gps fitness units out there, and make an informed decision, but my experience with the 310xt has been nothing but positive.