The size of the cylinder is very important--you do not want to get one that is too tight when you are pumping.
The pump system is fairly important, you do need to consider a gauge for the pressure- you do not want to over pressurize the cylinder and cause damage to the penis.
Get a good, comfortable sleeve that is a good fit for your girth. This creates the seal and there is nothing more frustrating than continually breaking the seal or a too tight fit. Tight is also dangerous to the health of your penis. Also, consider buying a silicone sleeve--safer material!!