lower end garmins, nike's stuff, etc are pedometer not accelerometer.
Garmin 305 is very accurate, but has a few design flaws, (speaker holes in the back are not waterproof, so over time they die due to sweat build up, generalization of course) but with that being said if it is under a year old they will replace it for free, and if it's over a year old its only 80 bucks to replace aka "repair".
The battery life is great, a full charge will last me 4 to 5 hours, i run 2 hours a day, so i only charge it every 2nd or 3rd day so the 310xt probably isn't needed unless your an ultra guy.
Dollar for Dollar i can not think of a better investment, becuase my first Garmin lasted me 2 years. I paid 150 bucks for it through Walmart, and i have used it for every run. Compare that to the price of shoes or anything else we runners buy, and cent per mile it beats them all hands down.
It makes you go a little OCD but when I travel out of town or even go exploring in town, its nice to know I'm still putting in the same amount of mileage/pace. Also if I go for a bike ride or hike it also has it's place.
If used correctly it can sharpen your Kung Fu, meaning if you run by feel, and only look at pace for gee wiz information, you start to know what your paces feel like, and over time your pace will drop but your exertion feels the same, which is a good sign of fitness.
With that you can go to the dark side and run every run at the same pace no matter how hot, windy, how many cheeseburgers you ate, or how much you slept last night, and if you fall into that kind of runner, well it probably won't work out for you.
Right now the 305 is so cheap, it would be foolish to pass it up if your interested.