ok... antisyphon = an angled tube that is fitted inside your co2 bottle which eliminates/reduces the risk of liquid co2 going into your gun.
expansion chamber= a chamber which allows this co2 from your tank to turn into a more vapour like form, thus decreasing the risk of your internals freezing up on you resulting in split orings etc etc yadda yadda ya. As the above person mentioned it would be a good idea for you to purchase both and use them in conjunction with each other .. hope this helped
As said above try and yes both, but make sure the anti syphon is done correctly for your marker (at the right point on the co2 bottle.
Also try and get as big a expantion chamber as possible (if I remember correctly they come in "stages" I had a "6 stage" one and had no probs what so ever, but I've known people to only have "4 stage" and have freezing up prob's because there not big enough, under fast firing conditions
get both if you want, does your gun already have a fore grip, if it does, just get the anti siphon, if you don't have a foregrip, get the expansion chamber, it's pointless to have an anti siphon and something that transfers liquid co2
A foregrip won't draw liquid CO2 if the anti syphon only lets gas out... Yeah a expansion chamber / foregrip is better as you will probably increase accuracy more from the extra stability the gun will have rather than whether it shoots a bit of liquid co2 or not!