The groupbuy price for the Blueflame is definitely very good, in fact its outstanding. The general opinion on the Milltek systems is that they're better built with better quality welds although I believe Blueflame have tightened up their quality control in this area. A Milltek for the
Ibiza has a 2.5" downpipe, this will give you a bit more torque but slightly less in the way of BHP assuming that both the Blueflame and Milltek cats have identical flow rates.
I was in the group buy at one stage but I got an offer I couldn't really refuse on a Milltek 3" downpipe, 100 cell sports cat and 2.5" non resonated cat-back system, £570 fitted. It worked out about £20 dearer than the Blueflame system after fitting is taken into consideration but for the peace of mind and outstanding warranty/customer care reputation that Milltek have, I've decided to go for the full Milltek system. Its getting fitted next thursday. In a couple of months when I get Revo stage 2, I'll see the benefit of the 3" downpipe, whereas I believe the Blueflame LC item is 2.5".
If I were you I'd go for the Blueflame as both appear to be 2.5" for the
Ibiza. A Milltek system will probably be at least £150 dearer.