If you plan on using heavy monofilament or heavy fluorocarbon line, such as over 20-pound test or heavier, you may want to consider buying a bait casting reel instead of a spinning reel. This type of heavy line will retain memory and come off your spool like a coil of wire.
It will become "springy" and bounce off your spool, rather than flow smoothly from your spool. And it will be very susceptible to tangling and not to mention it will be annoying to use.

Spinning reels are made for lighter line, with one exception. That is, if you use a thin diameter braided fishing line.

Braided fishing line is generally extremely very thin in diameter. Thus, in this case, you can use much heavier pound test line; so long as the diameter is not thicker than 20-pound monofilament.

Casting vs Trolling
Also, if you plan on casting with your spinning reel as opposed to trolling, then do not buy a large model. Buy a smaller reel so that you won't be so tired after a day on the water. A comfortable size spinning reel to use would be a reel with anywhere from 100- to 120-yard capacity of 10-pound test line. For saltwater fishing, you should upsize, to a reel which holds anywhere from 130 to 160 yards of 10-pound test line.

Balance
Moreover, your spinning reel should be perfectly balanced. When you spin the handle and let go, it should run smooth with no wobbling. Wobbling is a sign of poor workmanship and the reel will not last as long as a balanced model. Further, balanced fishing reels are much more comfortable to use as they will not strain the muscle in your wrist from all the wobbling.

Ball Bearings
As far as ball bearings go, generally the more bearings, the smoother the reel. However, I'd be hard pressed if I were blindfolded, to tell the difference between an 8 ball bearing reel and a 10 ball bearing reel. Personally, any fishing reel that has over 3 ball bearings in it will provide a smooth retrieve. After that, it becomes more of a personal preference.

Drag Systems on Fishing Reels
Drag systems are also extremely important. Front drag systems perform better than rear drag systems. This is because the front drag system uses multi-disc drag washers, allowing for a smoother and more durable drag system. The drag control is located directly in front of your spool, and the best drag controls will allow you to adjust in very small increments.

You do not want a drag system which releases line in "leaps" and is "jumpy". Your drag should release line smoothly and consistently when pressure is applied. This is referred to as a "silky smooth drag system"
Although there are other factors you could consider when buying a spinning reel, if you take the above into account when buying, you will have a good quality fishing reel that will do you well. The rest of the fish catching will be up to you!

In summary, use a smaller spinning real with a front drag system, and spool it with lighter line. Heavier line should be saved for bait casters.