I have a Garmin 705, but all the current Edge series use the same mount, and all Garmins use the same cadence/speed sensor.

The cadence/speed sensor isn't hard to install. It takes a few minutes to get it set up, and a work stand is a big help, but you can always just hold the bike up (or have a friend hold it) when you need the wheel to turn, and pedal backwards to make sure the cadence sensor is picking up. It installs with zip-ties, so it's plenty secure.

You don't need to worry about the 305 getting wet, it's IPX7 water resistant, which off the top of my head I think is 30 minutes at a 1 meter depth. It's not waterproof for swimming, and that's what's different about the 310XT, but all the Garmin heads are plenty safe for rain use. Water does tend to block the ports for the barometric altimeter, so you lose accurate elevation readings when it rains, but it's fine once it dries out.

What isn't waterproof on the Garmins is the cadence sensor. They need to be carefully protected when you wash your bike, and removed if you plan to ride in the rain. Carrying a set of nippers and a spare Ziploc bag, if you have room for them, is probably not a bad idea if there's a chance of getting caught in the rain. Garmin has replaced two of them for me under warranty no questions asked, but it's still a PITA doing without it while I'm waiting for a replacement (this summer, they were back ordered several weeks), so I really try not to drown mine.

The handlebar mount for the Edge series also isn't the greatest. It's just a snap-in plastic clip, and it wears out after the computer has been mounted and removed, oh, I'm going to estimate something between 300 and 500 times. The Edge needs to come out of the mount to charge and sync, and then you may want to take it with you if you're parking your bike somewhere, so that's like a year and a half of use, or less. There are buttons on both sides of the head, so what can happen is the computer just flies out of its mount when you push one of the buttons. It's happened to me two or three times, happened to a couple of my riding buddies, and there are several people on the Garmin forums online who've broken their screens or otherwise bricked their computers when the mount failed. If you go for an Edge, I definitely recommend a backup retention system of some kind. (I have a lens cap tether on my shopping list...)

The Forerunner 305 has a different mounting system that I'm not familiar with. With my older Forerunner 301, I could leave the wrist strap on and use it for backup retention (although that mount never failed).

Both my Garmins are quite accurate in 2D - the barometric altimeter is DEFINITELY noticeable in the Edge, and adds hugely to its 3D accuracy. Having the speed sensor is mainly useful against spikes, which occasionally happens with any GPS from interference from power lines, etc. I don't lose signal unless tree cover is both very heavy and wet.

Be aware when you say "305" that Garmin has both Edge and Forerunner models badged 305. It sounds like zoom-zoom is using a Forerunner and nscrbug is maybe asking about an Edge. On the bike, the difference is going to be the number of display fields, the size of the display, and the barometric altimeter. There may be differences in how the two computers handle workouts, but if there are, I'm not familiar with them.