Well, the 205/305 are the older/legacy devices. That being said - I love mine...and can't justify the double price difference to upgrade to the 310/910

They are hard to come by now - Garmin has officially discontinued selling new stock - Costco/Sams might have them new. (205 = 305, but different color and no HRM)
If you live in a really rainy/precipitation heavy, the 310/910XT would be worth it. My under warranty 305 shorted out last week (battery) - Garmin has excellent customer service and shipped me a replacement via 2 day, but made me be a bit more conscious of drying it off after runs/quick wash. I've never had issues with it in the rain before, so think I just got unlucky. (And/or it hadn't truly dried off before I put it on the charger to charge)
310xt = the new version of the 305, water resistant to 50M. Not truly a water/swimming watch, as it only estimates distance traveled.
910xt = the true triathlete's watch - measures actual distance, stroke rate, pool lengths, etc.
In regards to an everyday watch - none of the above would be suitable for an actual watch to wear around really. IE, battery life would prohibit it from a length standpoint, not to mention constantly searching for satellite signals (if inside). I wear a watch 24/7 (in the shower, in bed

, etc) - I don't find it a huge deal to swap watches in the morning. I had the same thought as you about wearing it as an everyday watch - initially when I started running, I bought a Polar RS100 HRM watch. It was great in that it was waterproof, could use as an actual watch/replacement for my G-Shock, was on all the time, and had a HRM. It sucked as a running watch though, because you honestly don't know how fast or far you are going, or can't realy truly analyze mile splits/times later on.
Honestly though - the GPS watch is the single most effective training tool you will ever purchase, and it pays for itself. (It costs what 1-2 pair of running shoes cost, or multiple pair of shorts, etc) My first Forerunner 305 lasted me 3.5 years. It was having a hard time finding a satellite at times, so sold it and got another 305 in Dec, which just shorted out. Replacement so far worked well, at least in today's 10 mile sweat drenched tempo.
NOTE: there are some running purists now that are anti-GPS watch - they think we've become too reliant on technology, knowing every stat, etc. I still go out on easy runs with a normal stopwatch and run based on time, but for actual half/full marathon training, it's invaluable. You can upload your runs/stats to MapMyRun.com, Garmin Connect, or the other lesser known sites. (RunKeeper, RunAhead) If you're a numbers/statistics geek like I am, you'll enjoy the fact that you can analyze it.
Ex, here is my NYC Marathon route from November:
http://www.mapmyrun.com/workout/76582888