Buying first hybrid bike; better to put $ into components or disc brakes? I'm looking to replace my 2008 comfort cruiser with a fitness style hybrid. My budget allows for maxing out at $500 (on the new fitness hybrid bike itself). I ride rails-to-trails with the kids, charity rides, and would like to commute to work on occasion (15 miles round trip). 

Ultimately the decision comes down to bike feel, but with the limited budget, am I better to put my money into upgraded components or disc brakes?
Typically, you don't need disc brakes on a bike which lives on the road or other nice surfaces. The primary advantage on the road is that you don't have to deal with wheel true-ness as much, and that they look cool. For wet weather riding, they can stop you a bit quicker, but with properly adjusted V-brakes, you can stop pretty damn quickly once you learn to feather the brakes. Discs also cost significantly more than rim brakes, and lower quality disc brakes (which you'll get in the ~500 dollar price range) will perform about as well as decent rim brakes. Your rims will last longer with a disc brake, but this is rarely to never an issue with a hybrid rider who doesn't put very high mileage.

The disadvantages of disc brakes are many -- for example, the front fork will be heavier since it needs to be more robust to handle the forces of disc brakes, and they weigh more than most rim brakes (weight weenie-ing isn't a big deal on a 500 dollar bike). Also, you'll have to take a lot of money (probably 20% of the budget) for the disc brakes, versus well less than half of that for rim brakes. 

I think this article from John Allen makes a good summary with a nice point-by-point list of advantages and disadvantages. Generally, for most people, the rim brakes in this price range are the better choice and enjoy the better components all around (these days, you'll probably get 1 level up in the drive train and a carbon fork, which is nice). And your dollar will go a lot farther on a used bike than a new bike (another post recently described a used Trek 7.5 FX for ~450 dollars, which would be a fantastic non-disc option for a hybrid). 

If you do want to get discs in the future, you do need a disc compatible frame and fork (excluding solutions like BrakeTherapy), as well as hubs on the wheels, so if you think you'll want them in the future, you probably should just buy the disc brake version (or buy the non-disc version, sell it and then buy a disc bike). 

Side note: Hybrids generally have two distinguishing options between models from a given manufacturer: with/without suspension and with/without disc brakes. Generally, you should choose no suspension (unless your doctor tells you otherwise (i.e. you have severe back problems)) and fit decently large tires, and without disc brakes since for most riders they wont see an advantage unless the disc brakes are high quality and their riding conditions require it. Usually, cheaper hybrids run mountain bike groups on a flat bar setup, while more expensive ones run road bike groups on a flat bar setup. You also get carbon forks after you spend a bit of money, which is nice.