I just got the Canon Rebel T1i and I love it. I previously had a 1st Generation Digital Rebel, and previous to that, film bodies from Canon, so I speak from a Canon expertise with a general knowledge of Nikons.
Some general advice - you can't go wrong with Canon or Nikon. Buying either system gives you access to a large range of lenses, on top of which the better lenses have great resale value. You can also rent gear if you are considering a particularly expensive lens purchase. One of the main points of buying an SLR is to have access to these lenses, so you want to buy one of the systems that makes a full set of lenses. If you are already using a P&S from Canon, you will find the Canon SLR's easier to move to. I'm not sure if the same applies in the Nikon line. You will find that an SLR gives you two things over a P&S, quality, and less thought about - speed. Trying to catch a squirrel on the ground? Baby crawling? Funny tourist moment? You will have more success with an SLR because it does everything so much faster.
If this is your first SLR, I would *not* spend big dollars on a body. Buy an extra lens and in three years time then you may want to consider spending more money on a semi-pro body when you know what you want from experience with your first camera. The more expensive bodies are also bigger. You'll be traveling, so you want a smaller body. Forget about mega pixels. It doesn't matter anymore if a camera has 10MP or 15MP. What matters is ergonomics, speed, low light performance, and preview quality. You need to be able to get a good preview to decide if you need to adjust your settings. You want good low light performance so you don't need a tripod to take that twilight scene in. Ergonomics is a very personal thing, and frankly, you won't be able to get a total handle on that just playing with the camera in the store. My opinion is that most people end up with Canon or Nikon based on that first impression in the store, and never look back after that because they get used to that system. I know I'm like that.
I'm quoting prices looked up today from
The Camera Store. Generally they have the best prices in Calgary - I cannot vouch for their service, as I buy my gear at
Robinson's Camera - they match prices.
In considering the Canon line, you are looking at the Rebel line. The XS with 18-55mm IS, is $540 and makes a fine first SLR. It's downside is that it's a previous generation sensor, and is not as good for low light noise, and the display screen is not as nice as the newer cameras. The next step up is the XSi, $700 with the 18-55mm IS. It's last year's model and compared to the XS, has a bigger preview screen, and slightly improved sensor. The latest rebel is the T1i at $990. The sensor is again slightly improved, with a better preview screen (a full mega pixel, very nice preview quality). If you would like to do some video with your SLR, this is the first model that can do that. Personally, if I was on a limited budget, I would get the XS and use the $450 towards a better second lens. If you really want video and like that great preview screen, go whole hog up to the T1i.
In trying to decide which Nikon to get, read the
recommended camera article by
Ken Rockwell. His site convinced me I had to get a wide-angle lens this time around. I will only add that for travel, I would take a close look at the D5000 with the flip down preview screen. With live view (preview on the screen rather than the viewfinder) and the screen tilted down, I can hold the camera over top of a crowd to get shots I normally wouldn't get. I totally miss this feature from my days of shooting with a Canon G2.
In considering my purchase of the Canon T1i, I spent about 10 times the amount of time considering lenses over considering the camera body. If you are going to consider Sigma and Tamron 3rd party lenses along with the brand name lenses, you have even more to consider.
First the "kit lens" The Canon 18-55mm IS is a perfectly good lens. Nikon makes something similar, and I've heard good things about that. The days of kit lenses being garbage are gone - I consider them to be about mid-level these days. You'll want IS (Canon) or VR (Nikon) as it steadys the shakes that blur photos shot in low light. You may already have this on your P&S in fact. Since you'll be traveling, you will appreciate the kit lens light weight, and the IS will help you get more photos of non-moving subjects in low light. On top of all that, the kit lens is a bargain, for Canon at least, it adds $100 to the price of the camera - you will never buy a lens that cheap again.
If you decide you want to take another lens along, you can go one of two ways: one would be to ditch the kit lens and get something with a wider range like a 18-200 IS ($663, The Camera Store). It's a pretty common option as a one lens does it all option. You pay a price though in that the quality will not be as good, the speed not as fast and low light performance about the same as the kit lens. Keep in mind though, when I say the quality will not be as good - it will still run circles around your P&S. For travel, not switching lenses means more time concentrating on taking pictures.
If you go with multiple lenses, consider the following - what you pick will depend on what kind of photos you like to take. Also consider that once you decide to spend big $ on lenses, you are really committed to a system, since selling all that and switching will cost you.
Wide angle zoom - Canon 10-22mm If you want dramatic scenery and building shots, this is the lens to get. I own this lens and so far have used it to great effect shooting downtown - not cheap at $995. Sigma makes a similar lens for slightly cheaper at $700. You will find yourself challenged with this lens - ultra-wide angle takes some getting use to.
Are you going to be shooting animals at a zoo, or not too far away in the wild? Want a dramatic sunset shot? Isolate detail in buildings? Compress the foreground and background? You want a tele-photo lens. The sky is the limit in terms of what you can spend here. You will want something lighter for travel. In the Canon line, I suggest the 70-300mm F4-5.6 IS ($798), being used here on this forum with great results. It's still relatively light and will out-perform the 18-200mm lenses at the ranges it overlaps. I'm going to recommend you stay away from the 55-250mm F4-5.6 ($350), despite it's low price. I'd rather get an all-in-one 18-200mm at that point and not worry about switching lenses as the quality difference is not great enough to bother in my opinion.
I spent about three months doing research before I purchased my gear. I wrote up a blog post about the sites I used to research my purchase decision, read it
here.