Things for Carbon rim:
Braking performance. Early carbon fibre wheels were pretty dire when it came to stopping, but manufacturers have engineered their way out of those early problems. Much of the problem had to do with heat buildup. A problem for early carbon wheels, great improvements have been made with managing heat, from the type of resins and carbon-specific brake blocks used. The latest carbon fibre wheels now offer a big improvement in braking performance compared to those early designs. Still, a good aluminium rim provides better performance, especially in the rain. Aluminium deals with braking heat well, carbon doesn't so well.
However, the rise of disc brakes has made this yesterday's problem, provided of course you have a disc-brake-equipped bike. Without a brake track, rims can be made a little lighter too.
Weight. While carbon fibre tubular wheels might appeal to weight weenies, the fact is that the cheaper and more common carbon fibre clinchers are typically the same weight, or heavier, than a good quality aluminium wheelset. If you want the lightest wheels on a budget, then aluminium is still the way to go. The Spada Stiletto wheels we tested a few years ago weighed just 1,290g yet cost £700, cheaper than most carbon fibre wheels. However, you do have to factor in the aerodynamic benefits of a carbon rim over an aluminium rim when comparing the weight.
Real-world aerodynamics. Manufacturers make impressive claims for the aerodynamic efficiency of carbon fibre wheels, but how that translates into the real-world with a vast range of conditions is questionable. Factors such as tyre size, frame design and wind conditions make a big impact on any claimed drag savings. Another important factor is that deep carbon rims can be very unstable in strong crosswinds, especially for lighter riders. Shallower section carbon rims (30mm) have become popular for that very reason.
Durability. Carbon is fantastically strong stuff and carbon fibre wheels can be impressively durable. They don’t bend, though, unlike aluminium, which means they can be prone to damage from sharp impacts or crashes when they’re subject to loads they’re not designed to cope with. While a bent aluminium rim can be straightened, enough to get you home, it’s not the same with a carbon rim, which will most likely be a write-off. That’s why it’s always worth buying a wheel with a decent crash replacement policy.
So, carbon fibre wheels are clearly very popular these days, with more choice and lower prices than ever before, but there are clear pros and cons. If you’re racing and can afford them, carbon fibre wheels are easy to justify, but if you’re not racing and value durability, braking performance and affordability, there’s still life in traditional aluminium wheels yet.