I'm going to assume that anyone with pro experience or realistic aspirations to become one is going to be reading about this elsewhere. The upper-range DSLRs are large, heavy and demand large expensive optics to show their image quality advantages. They end up in large, heavy camera bags and turn trips from family exploration to photo-safaris.
|Canon 3Ti and kit lens|
There are better kit lenses, fast lenses, cheap longer telephoto lenses (with a kit, an extra $115), external flashes, and much more. The more you get, the heavier the bag will be. For light travelers, the only extra lens I would add is the Canon 50mm/1.8 to allow for low light situations ($105).
The next step up begins to edge into diminishing returns. Here something like a Canon 5D Mark III with its 24-105 kit lens is perfect for travel. Price leaps to about $4,000 (four thousand). For this you get a slight improvement on image quality and really good low light performance. This is at the low end of the pro camera range, but in the right hands eminently capable of most pro tasks. I would add at least a 35mm/2.0 lens (about $300) for low light use.
Besides the expense, this can quickly balloon into more bulk and weight than one might want to carry on vacation. Changing lenses on a DSLR introduces dust into the camera and sensor.
The camera bag will need to be bigger to accomodate the larger camera and lens(es). Make sure it allows you easy access to the camera.
This is well into overkill territory for a vacationer, but for some just the ticket. Make sure you want to carry that much weight and bulk on your trip. Don't be the guy who's snapping away while your family sits around bored, waiting for you to finish.
Note that I'm not mentioning accessories. They add up quickly in terms of expense, bulk, weight and they slow you down. If your trip is primarily a photo expedition and everyone going with you is doing the same as you, fine.
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