I would suggest a bike that you dont mind locking up outside (go used, inexpensive if you can). This bike can be utilitarian since your commute is quite short. Make it fun though, and you might find yourself taking it other places.....
For the trail riding, I would suggest visiting a LBS and asking them what is available in your price point. Try not to be seduced by fancy derailers etc. A bike company may install a nice drive train, but it may be at the expense of the headset and bottom bracket. As well, for that price, you might consider a bike with a nice frame and front suspension only. If you have a choice between a good frame and average componants, or fancy stuff on a cheap frame, go with the better frame everytime. (Please keep in mind I do not know what 500 buys you in the USA, up here in Canada, it does not get you much.....)
Being a road nut and mtn bike nut, I personally find that bikes that claim to do both rarely succeed. However, since your commute is short, and if you have to get just one bike, go for a trail machine and ride it to work. What to do depends at least in part on how secure your bike lock up is at work. If it is outdoors, a cheap disposable bike might do the job, but if it is indoors, then a more expensive option might do the trick. (In my opinion, a better bike is one that is more suited to the job at hand: my mtn bike is NOT a better bike for commuting vs a single speed old ccm with a coaster brake, rack, and fenders. But is is much superior for carving up trails.)
Good luck, and ask a local person as well what kind of bike would work best on the local trails.