The principal practical difference between these is duration of action. There’s also some difference in the amount of actual testosterone contained per milligram of drug. In order of highest testosterone concentration to least-high but still high, the order is suspension, propionate, enanthate, and cypionate.
In practice however, I suggest forgetting about that! Rarely does anyone account for this in deciding their testosterone dosage. It’s a point that may be glossed past with no problem. The above is also the order of duration of action, from shortest to longest.
For practical purposes, testosterone cypionate and enanthate may be used fairly interchangeably. The half-life of enanthate is probably about 4-5 days, while that of cypionate is probably about a day longer. Accordingly, both of these clear the body relatively slowly at the end of a cycle, causing a relatively long period where levels are neither high enough to allow much if any further gains, yet not low enough to allow recovery. Testosterone propionate has a much shorter half-life of probably only about 2 days. As a result, clearance of testosterone propionate is quick at the end of a cycle.
By half-life, I mean the time period in which levels of drug drop by 50%. For example, if a drug has a one day half-life, then after one day levels will have fallen to ½, after two days to ¼, after three days to ⅛, etc. Testosterone suspension has a variable duration of action of at least several days and up to at least a week. The pattern in which levels drop, however, is different from those of drugs having a half-life. Instead, particles of drug slowly dissolve, and the particles slowly shrink. Release slows somewhat as the particles shrink and their surface area decreases. Any given particle will at some point disappear entirely. When nearly all have entirely dissolved, the duration of action of the suspension has ended.
Because of how this works, duration is dependent on particle size of the suspension. This is variable between products. Broadly speaking though, products which are milkier, which take longer to fall out of solution, and which go completely easily through the finest needle will have shorter duration of action than coarser products. A good suspension of testosterone can be used similarly to testosterone propionate.
I’d recommend saving the propionate and suspension for use at the end of the cycle, to enable faster transition from high levels allowing gains to low levels allowing recovery. There’s no value in spending an extended time after the last injection with levels still too high to allow recovery, yet not high enough to allow further gains. Actually, it’s not just that there’s no value, but a negative value: it impedes recovery. Strategic use of propionate esters and suspension at the end of a cycle aids rapid recovery.