Commuting DHB, disc brake, rack compatible ~$600
 

I'm pretty new to cycling but I'm ready to get a more serious bike since my last one was stolen a couple years ago.

I'm hoping to find a solid bike to last me 3-5 years. After that I don't know what I'll be doing and as such my budget is not too serious ($500-600).

Features I'm looking for:
Dropped handle bars
Disc Brakes
Rear Rack Compatible
24+ speeds
Optional 32cc tires

I'll be using it to commute a few days a week, 8 mi. I'd also like to ride on weekends 20 - 30 miles at a time, maybe even tour more of Southern California when I have time. My town (San Diego) is not too hilly, but I live on a pretty steep hill, and I also want the flexibility to cut over grass or looser gravel, so I'm thinking 32cc tires would be ideal.

I weighed the Marin Lombard 2012 for a while, thinking that had everything I need, but it's rather expensive compared to some bikes on Bikes Direct.

I was debating between the Motobecane Turnio and the Gravity Liberty CXD but I don't have the knowledge to tell whether the specs on those bikes are far inferior to the Lombard, or whether 3-5 years is a reasonable lifespan for the cheaper bikes.

Any advice is much appreciated! Thanks for your help.

To get those criteria in a new bike at that price-point, I think you'll find BD to be the only game in town. There's nothing wrong with either of the choice you're looking at. If there is a weak point on any of the specs, it'll be something that you'll just upgrade as it wears out (i.e. chain, cassette, maybe RD).