Been covered many times, but in a nutshell :
PSI = Pounds per square inch, i.e. the pressure the air is stored at. Usually available at 3000 or 4500psi (though a few 5000psi systems coming available now). You can put 3000psi into a 4500 bottle but NEVER EVER the other way round or it will go bang in a rather spectacular manner. Obviously, if you can pressurise your air more in the bottle you can get more air into the same space, so will get more shots from a 4500psi bottle than a 3000 of the same size. 4500psi bottles are more expensive.
CI/CU = the size of the bottle. Usually comes in 1.1, 1.5 or 1.9 litre capacity, which roughly translates into (I think) 68, 88 & (I can't remember, probably around...) 108 cubic inches. The bigger the bottle, clearly the more air you can fit in but the heavier and bigger so less easy to use.
Unless you have a phenominally gas-hungry marker or are a paint slinging back player, a 1.1l bottle at 4500 psi is probably your best bet.