Good unit. Have not owned a 305, so I don't know if it's worth the extra money.
I would have kept it even though the software is was painful to get started. I might have even kept it if the HR was in the range of being somewhat accurate but when the bezel kept locking up and I couldn't track my runs I almost threw it in the trash. I regret buying this product.
I love it and I would recommend the Forerunner 405 to anyone! I bought the heart rate monitor and cadence monitor separately at a discount on Amazon and haven't had any issues.
When it works right its awesome but so often it "freaks out" and won't work right.
If you have smaller wrists, this would probably be an excellent device for you. If you have larger wrists, I would stick with the FR 50 unless you absolutely need the GPS features.
I really do like this watch. I had borrowed a friend's 205 for a month while training for my first tri and it was very useful, but way too big. I actually bruised my wrist bone from it hitting it while running. I'm looking into getting the heart rate monitor as an add on and perhaps the bike cadence sensor as well. I'm sure I haven't used all the features and explored it to its fullest potential yet, but so far, I am really pleased with this. It was a gift and it won't be returned.
I've had the watch for a couple weeks now. I used it in a race last Saturday and it performed great. I've also used it on a 5 mile run and it seemed like it was pretty accurate. I'm planning to purchase the bike cadence and foot-pod accessories soon.