I posted this in the 'Who is Hambini' thread also:
AC bearings can last longer in BB30 applications because they have higher load ratings than radial ball bearings. This is down to the filling method of the radial balls (look up Conrad filling method). So an AC of the same size will have more balls and a higher load rating than a radial bearing. So basically nothing to do with axial loads and/or preload. AC has slight rigidity benefit too as the 'effective centres' of the bearing are further apart. Forces are applied to the bearing where a line drawn through the contact angle bisects the axle centreline. However AC is only effective with fixed mechanical adjustment (e.g.screw thread). If your BB is preloading the bearings with a wavy spring then it wont be an AC under a radial load.
Now if you take a radial bearing with a higher radial clearance (C3) and apply preload it becomes an AC albeit with a low contact angle (typically 10~12deg). A bearing with lower clearance or 'standard' clearance CN will have a lower contact angle when preloaded. So C3 clearance will give you a marginal theoretical benefit but misses the main advantage of an AC which is the higher load rating. However the extra clearance in a C3 can be helpful if the machining of your BB is not very accurate and the two bearing positions are misaligned relative to each other.
OK so now lets have a look at precision bearings. Actually the P classes are ISO (Euro & Asia) and ABEC are ANSI (USA) but they are directly comparable P0 (ABEC-1) is standard then P6,P5,P4 & P2 (ABEC-3,5.7 & 9) are all improvements. Now consider that a BB30 bearing size 6806; the standard clearance, standard precision version of which is rated to run at max 15,000rpm and will do so all day long if so required. Now consider that the main benefit of higher precision bearings comes when you wish to exceed the max speed rating of a 'standard' bearing. Then consider that very few cyclists can get anywhere near a 200rpm cadence (yes that is 1.3% of the available speed capability!) Also higher precision bearings demand higher precision from the mating axle and housing journals so fitting them to standard parts has no benefit at all.
No matter how fast you think you are ALL BEARINGS IN BICYCLES OPERATE AT LOW SPEED. That is also why ceramics offer absolutely no tangible benefits for cyclists.
More than 50% of the drag in an unloaded bearing will come from the seals. This will reduce to less than 50% under load because mechanical drag is proportional to load and seal drag is constant. Now the 50% figure will apply to full contact seals (for example NTN seals have two contact lips at each side - which is why they last so well). This can be reduced by using low friction seals (single contact lip), non-contact seals or metal shields. There is no getting away from the fact that seal drag and sealing efficiency are directly related. As soon as you get any dirt inside a bearing you will massively increase the drag torque. This is down to the tolerances and clearances; again using the BB30 size 6806 as an example a CN version will have a radial clearance of 5~20microns (1 micron = 0.001mm) the C3 version 13~28microns. Now consider that a human hair has an thickness somewhere between 50 and 250microns; probably best to keep your helmet and shorts on when servicing your bearings!
If you want to give your cartridge bearings the best start in life try running them in before you fit them. Because bicycle application speeds are so slow the correct grease distribution does not happen. Therefore fix up your electric drill to spin them for 10 minutes at a few thousand rpm. This will help distribute the grease which has many benefits; reduced drag in bearing and seal, improved sealing efficiency, better corrosion protection & better lubrication.
Apologies if this is a bit heavy going but hopefully some of you will understand the key points here. Regarding the OP question any quality bearing is preferable SKF,***,NSK,NTN. Note that in terms of sealing efficiency the NTN full contact seals have a 2 lips on each side (NTN suffix LLU). These are by far the best options for keeping dirt out in my experience. You can halve the seal drag by removing the inboard seal if the BB housing bore has no access holes from the frame tubes.