Any carpet is only ever as good as its underlay. Buying cheap underlay or keeping old flattened underlay is a false economy.
The trendy textured waffle-weave type carpets can flatten very quickly in high traffic areas if you buy them too cheaply, and they tend to be 100% wool, which can be tough to keep clean. They do look nice though!
A decent all round timeless classic is the 80:20 wool twist, or a berber. (with flecks) Looks plain and boring but that's no bad thing, as it means it's more versatile in the long term. 40oz is quite thin, 50oz is adequate, 60oz is thicker, (and a denser pile) and therefore better. But if you've got great underlay you can afford to risk thinner carpet!
The higher the wool content, the nicer it will feel, but the harder it will be to maintain/keep clean.
If you are looking for a neutral colour, remember it always looks lighter when it's down that you think it will. So don't be tempted to go for a very pale shade.
Natural sisal looks beautiful and is incredibly hard wearing, but is expensive/complicated to fit, and an absolute pig if you spill anything on it, or have wet muddy shoes on it. Would be disasterous with small DCs.
Almost always, the existing gripper rod is fine, don't get conned into replacing it - they charge a fortune even though it's just scrappy bits of wood. The only exception is if the spikes on the existing rod are too long/short for the thickness of the new carpet. Too short and they won't grip, too long and you'll feel them through it - not good!
My best tip would be to remember that there is no such thing as a free lunch. Equally, there is no such thing as free fitting! They just build the cost into the square meterage of the carpet, so you are paying more for the carpet than you should be. The danger with this is that it fools you into thinking it is a better quality carpet than it really is.
Also, these superstores that do permanent sales (50% off certains lines at any given time) are a con. They mark the prices right up (way beyond a fair price for the true value of the carpet) for a few days, then mark them back down to a fair price, and tell you it's a half price sale.
They also say that you'll not find the same carpet cheaper, but that's because you won't find the same carpet. They have the big manufacters make stuff just for them, under a different name, so you can never do a direct comparison with another shop.
It's infuriating because they may have as many as ten or twenty carpets which are all basically the of comparable quality/type/weight at any one time, all sold for completely different prices depending on whatever dodgy offer they are touting that week. It makes it impossible to evaluate the true quality of one carpet against another. (you may have gathered I'm not a fan of these shops!)
I would recommend you look at either:
A good small independent retailer
John Lewis (never rip you off, always an honest pice, good quality products, excellent service.)
Look in superstores for good quality room size remnants, buy a bargain, and get a local freelance fitter from the yellow pages/local ads to supply underlay and fit.