Any recommendations on a good tripod--possibly two: one light and portable and another robust? What should I look for in terms of build? brand? typical cost?
Volumes have been written about this subject (and I'm about to add to it)
Besides recommendations here, be sure to read up in other gear related forums.
POTN is a great resource for Canon shooters (although their accessories forum is valuable to anyone). I assume Nikon fans have a similar forum you can leverage if you go that route.
I initially had two tripods, just like you say... one light for travel, the other one built like a tank for use out of the trunk of my car. If I only knew then what I know now.
Long story short, I believe you can get a tripod now that will cover both tasks without compromise.
Popular tripods basically come from either Italy or Asia...
- Italian products include Gitzo and Manfrotto. Well known and respected products with Gitzo clearly at the top of the price range. Manfrotto is more of a working professional brand.
- Respected Asian products include Feisol, Surui, Benro, Giottos. Recently these brands seem to have come into favour with enthusiasts for having outstanding specs, build quality, at comparable if not better price points than their Italian counterparts.
- There's also Really Right Stuff in the USA (also premium priced).
I wouldn't say any of these brands is superior to any other. You can find threads of broken tripods for any and all brands.
They can be made of two materials...
- Aluminum is cheaper but slightly heavier
- Carbon Fiber is more expensive but lighter
They come in a variety of sizes... with the key considerations being...
- How compact is it when folded? Will it fit in your suitcase? Your backpack?
- How high is it when extended? - without the center column since you want to avoid that in all but the most extreme situations
- How many leg segments are there (avoid 5 segments) and how thick are the legs (both first segment and last segment) since this can influence stability.
In my case, I wanted something that collapsed under 20" to fit in my suit case, was light, extended to my normal eye height without using the center column, with the biggest diameter legs possible. I ultimately zero'd in on the well reviewed Feisol and the legs that met my requirements were their 3441T. What's best for you will depend on your particular size/weight requirements.
When it comes to ball heads, every tripod manufacturer makes their own heads in a variety of sizes and weight bearing capacities. The general rule of thumb is to multiply the weight of your gear by 3 when evaluating the holding capacity of a ball head. But I would basically ignore the load specs, and focus on the ball diameter. 35-40mm is probably ideal for an average SLR/lens combo. Smaller and you'll likely have droop, larger and you're just packing more weight than you need to. The other key consideration in a ball head is the clamping mechanism. The most popular these days is the Arca-swiss style which has been widely adopted by many vendors so there are a ton of relatively interchangeable plates and clamps to choose from. Markins, Photoclam, and Benro are well respected enthusiast ball heads.
In my case, I went with a Markin's Q3T ball head with Arca-Swiss style Quick Release. It offered great capacity, small size, low weight and is well reviewed. To go along with it, I got an L-plate off ebay that fits my 5D3 perfectly and allows me to quickly switch from landscape to portrait without moving the ball head into an awkward position (often putting the whole camera off balance).
So while I can't recommend something for you without better knowing your requirements, this is what I ended up with that is both super light, compact, easily stowes in my suitcase, and sturdy enough to withstand a tornado

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- Feisol CF tripod legs (eg. 3441s or 3441T depending on height)
- Markins Q3T ball head with QR clamp
- Generic chinese L-plate off ebay
If I can offer one piece of advice... it's similar to what I mentioned earlier about camera bodies... if you can afford it, skip the step of buying cheap crap now and just buy a top shelf tripod and ball-head right out of the gate.

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The lower end camera drive me batty with having to make all the adjustments thru menus on the LCD. The mid range camera add a few more buttons to the back and am extra dial on the front and most adjustments can be made with the camera in the shooting position, instead of staring down at it in your lap.
This is absolutely true. In the higher-end cameras, you're getting a lot of added convenience along with the potential for better images.
I would purchase frame only (kit lenses are junk) and save the extra to buy high quality glass. That is what will make a huge difference in the quality of your shots.
Another benefit to going with a higher-end (FF) body is that the kit lens is actually respectable... with Canon you get the 24-105L which is a fantastic lens.