I've lived in both a downtown neighborhood and a west surburban neighborhood. The Jeffco Open space MTB trails are all over the foothills. If you live in Lakewood, Golden or Morrison, you'll be 10-15 min away from a dozen trailhead choices. If you're downtown and close to 6th ave, you'll be 30 min away. Granted, it's not a big deal, but when you're on the west side and *so* close, it really is nice. The only catch - if your sister has a job within Denver city limits, the AM eastbound and PM westbound commutes are heavy. I'm lucky enough to work on the west side, so even when I lived downtown, I was commuting against traffic. If you live on the west side, on Jefferson County, you can have easy access to all the great outdoorsy stuff in the foothills, and still be only 20-30 minutes away from downtown. The closer to 6th Ave, the easier/faster it is to get anywhere. Cheers!