Basic agreement here. I've spent the bulk of my life in the DC area. I grew up in the Maryland suburbs & have lived in the city proper since 1991. To put things in perspective, while crime in DC hasn't dropped as precipitously & quickly as it has in NYC, which is now 1 of the safest cities in the world, the city has steadily & noticeably improved in the years I've lived here. DC is probably the safest it's been since the early 1960s. For the most part, just as w/almost every large city in the world (particularly true of capitals) any neighborhood that interests or attracts tourists & business visitors will be very safe. Mostly for economic reasons, Baltimore's crime rate remains significantly higher than DC, but the same basic rules apply. Even during the worse days of the 1970s-80s, law-abiding commuters & residents ran a higher risk of being seriously injured in a traffic accident in the suburbs than being a victim of violent crime in the city. The bottom line is that safety/crime concerns for visitors to urban America are often greatly exaggerated by many Americans (who mostly live in suburban & rural areas).
As far as recommendations on equipment for the OP, much obviously depends on the type of photography that interests you, e.g., the requirements for street shooting being very different from landscape, sports/wildlife (zoo), or close-ups of flowers (FYI, the cherry blossoms won't be peaking for at least a week or 2:
http://www.nationalcherryblossomfestival.org/about/bloom-watch/). Any of the equipment combos you've listed can work well for general tourist snaps of monuments, people, etc., but I would guess that the Nex3 has better close-focusing abilities if you're planning on doing any significant amount of macro.
To echo bigeye's first point, to the extent that your choice of focal length/field of view is heavily influenced by physical distances, then you should know that most cities in North America, even much of relatively dense places like NYC & Chicago, are much more spread out & less dense than HK, Shanghai, etc. Because of height limitations & planning, DC is much less dense than other pre-automobile era cities of similar size, like Baltimore. Outside the monumental core, even the most dense residential neighborhoods are typically made up of 19th century row houses (more like much of Boston and Philadelphia) than apartment blocks in Manhattan & many residential areas relatively close to the city center would be considered positively suburban in other parts of the world (houses w/yards & driveways, etc.).
Have a great trip (I might even inadvertently run into you here in DC while I'm out shooting)!
Shuo: DC will have some longer shots that most cities - monuments/landscapes down on the Mall. It's quite open. Baltimore is regular "city distances".
I lived in DC for 18 years and are only a few places that anyone would have trouble. I work in Baltimore a lot now and it can be a little rough at night downtown on the edge of the tourist areas, but no real worries.
Like any city, it's how you handle yourself.
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