It's definately a tough choice.... being happy and actually enjoying what you are doing would probably be the most influential thing that formulated my decision.
Next, I would ask myself if I was consistantly learning something new in my current position; the last thing you want (or I want) in a position is to let boredom and redundancy to settle in.
Also, do you think you'll be more excited about a position in the new company or your current company? Meaning I suppose, after a long day at work coming home or going to the pub and brag to the wife or good friend or someone hanging out at the bar about what you've done that day or what you will be doing (being sure to not break any nda's of course).
Does the job fit your lifestyle? If you have a family you more than likely would want the flexibiliy to take off and take the kid to 6:00pm soccer practice or whatever interest they may have.
For me it's: Happiness, education, excitement and lifestyle.... of course, I'm 37 and have been through the whole gammet, sold my 1/3rd of the company I helped start because boredum, redundancy and stress settled in so I'm taking some time off, relaxing, talking to people and hopefully something will fall out of the sky.
OH, whatever you do... don't burn any bridges! If you're going to take the other job explain to your boss why you made the decision and if he's a good boss he *should* understand perfectly and keep the door open.
Good Luck!