Just buy an adaptor for the plug, i picked one up easily for a solenoid i bought from Germany but not from this seller, the adaptors are widely available i have seen them in several places since.
Both look good although the main difference seems to be the reactor or diffuser, i think an external reactor will work better for your tank, this is plumbed inline on your external filter, the glass diffuser may not work so well for a 100 gallon tank, you may get uneven distribution of the CO2 throughout the tank. You can also build your own external reactor as well if you wish, there are lots of plans on the web if you do a search.
I dont know if its worth auctually buying the cylinder from the ebay seller unless you know somewhere that can fill it for you, you would be better off finding out through your yellow pages etc somewhere that auctually does CO2 refills and buy a cylinder from them directly and then you will be sure that they will refill it for you, or a fire extinguisher is also another option.
/www.dartfordaquatics.co.uk/product_...5&cat=33&sub=60
