magnex have gone bust, they should be lots of them about going cheap. esp on ebay
ive got a milltek 2.5" resonated system from a astra gsi
i changed it before fitting cams, it pulls harder going up hills, but otherwise feels a bit flat.
i took the drastic measure of fitting a jetex restrictor, its now nearly o.e silent on idle, but feels more lively, like when i had the standard 2" system on.
i think your choice depends on what you want to do to your system, if your going all out then the bigger system (2.5") will make good sense, but if not then maybe the smaller 2.25" might be better.