One of the things you must keep in mind is the pump as well. Make sure to get something that is quality, easy to take apart and repair, and rated high enough to fill your gun, and has something to remove moisture. Some are rated to only 250 bar and others to 310 bar.
Also your shooting habits may dictated what you are willing to pump. I use a hill mk5 to fill a FX crown mk2. It takes about 25 pumps for 10 bar increase in the 480cc bottle. I use mainly for pesting, so I can get plenty of shots on a fill, and take my time refilling. I also have a Taipan Veteran Long on the way which have a 300cc cylinder, and a 250(approx: 3600psi) bar max pressure like the Crown. So, larger guns can be filled by pump, but it's alot of work.
If you plan on shooting alot stick with lower fill pressure guns that get decent shot count like Marauder or similar.