There are four kinds of trackballs: thumb-operated, ambidextrous fingertip-operated, right-handed fingertip-operated, and "other" (there are many tiny embedded trackballs and some handheld ones, but they're generally hard to get or very poorly implemented). Each of them is quite a bit different. In fact, the only thing they have in common is the stationary ball.
Thumb-operated trackballs have the benefit of familiarity (shape and button layout similar to mice). They're all but one right-handed, and vary in size/slope; Logitech (TrackMan Wheel series), Microsoft (Trackball Optical), Elecom (XT1-XT3, XT4 is left-handed), Perixx (517/717), Sanwa… All but one (which is rare and extremely expensive) are consumer grade and have varying quality problems; old Logitech or Microsoft trackballs would be better, but those have outdated sensors. There's also a potential problem with using thumb—it might hurt the knuckles and what not.
Asymmetrical fingertip-operated trackballs—arguably the best ergonomic option—have been mostly long discontinued; this includes Microsoft Trackball Explorer, Logitech Marble FX (or Cordless TrackMan [FX]), Kensington TurboRing and a few others. There's only one in the market, actually, Elecom DT2 (or DT1), but it's too small for many people, and the execution is far from perfect.
This leaves us with ambidextrous trackballs: either basic smaller models (Kensington Orbit series, Logitech Marble series), or large ones (either consumer grade, such as Kensington SlimBlade and Expert series, or industrial, such as CH Products DT225 or CST trackballs). The obvious benefit is that they can be used left-handed, but you have to figure out how to use them and your posture overall. It's usually also helpful to play with sensitivity and acceleration curves, in part because the sensors tend to be outdated.
Search here (and at Deskthority) or check out
/r/Trackballs (including the wiki) for reviews.