Keep in mind that I am only one person and as such, I have not had the opportunity to try out every brand of field boot, but I do listen and watch and can therefore give a few pointers.
There are roughly three levels of field boots out there: the really cheap ones (say $200 and under), the mid-range ($300 to $500) and the high end ($800+). There seem to be a lot of boots clustered within these price points and there can be quite a difference in quality within a single price point. This means that there is no clear answer on which boot will work best for you; you need to weigh the pros and cons of what each boot can offer you in terms of quality and features for your particular budget.
Low-quality boots tend to use low-quality leather. This leather is usually comparatively stiff and the comfort level may or may not be decent. If your boots can stand up on their own without boot trees, the leather is extremely stiff. The reason, besides comfort, why we don't want stiff leather for a field boot is because we want an extra-tall boot that will settle and form wrinkles in the ankle following the breaking-in period in order to give the ankle lots of room to flex. With very stiff leather, the boot might have trouble settling and therefore might be uncomfortable behind your knee or the stiff wrinkles might rub your ankles. The wrinkles also might not form very smoothly, one of the reasons why a boot might not look 'A' circuit quality.
Another problem with the lower-end boots is that the leather might be of a rougher texture and therefore not take a shine very well, even after a good polishing. If there are lots of tiny crevices in the leather, it simply can't reflect light the same way a smooth surface can. Additionally, a manufacturer might make their boots more cheaply by sacrificing height, making it very important to try the boots on and imagine what the height will be after the breaking-in period (expect at least a 1 1/2" drop). In cutting costs, a boot that is marked as "tall" might be the equivalent of a regular height in another brand. You want the back of the boot (don't look at the outside of the boot since they are made with a curved top that should cover part of your knee) to finish just below the back of your knee after the breaking-in period, which means that you need that extra height extending up past the back of your knee when you buy them. Too-short boots look very sloppy in the show ring.
Additional manufacturing details like the quality of the dye used or the quality of zippers, etc., cannot necessarily be judged at the tack shop and so it is important to do research to learn how a particular model of boots has fared for others over time if you want the boot to last. After all, there's no point in spending extra money replacing a cheap pair of boots if you could have spent a little bit more in the first place for boots that could have lasted twice as long.
Mid-range boots can definitely be acceptable for 'A' circuit showing, provided you're careful to choose boots that fit and are of good quality. The leather should be more supple than that of a lower-end boot, have a smoother finish and overall more refined styling. You can also get some nice features like gussets to make the boots more comfortable and a bit more forgiving as far as what you're wearing or how long you've been on your feet.
I'd say that the mid-range is probably one of the most important areas for which to do your homework because the price point allows for a wide range of qualities.
The higher-end boots will have the most supple leather and the most sophisticated styling. Some say that the leather used by some manufacturers is actually too thin and soft and therefore will wear through quickly on the inside of the calf, so that's something to consider as well. Manufacturers at this level should be eager to please their customers, however, so you might get better customer service than you would for a lower-priced boot.
In a nutshell, it will vary from brand to brand and there's nothing to say that you can't find a suitable boot for the 'A' circuit on a budget if you do your research and are willing to look at a few different brands. If you're lucky, you might find a high-end pair in your size in a used tack store or on clearance. If you can, try to get out to the tack shops in person to feel the quality of the boots, because it's easy for the companies to make a boot look nice and shiny in a photo online or in a catalogue, and a photo doesn't tell you how supple the leather actually is or how comfortable the boots will be to wear.