I'm looking at buying a new cannondale supersix evo from 'wheelbase.co.uk' and want to know which groupset will be better for me. Its either sram red or ultergra di2 I understand you get more ceramic with sram red
Red all the way!
The SRAM Red is going to be much lighter being the lightest group set available. However there are fundamental differences between the three major manufacturers in the way they shift. My advice would be to ride a few different ones and see which you like best. Everyone I know that has gone electronic say they would never go back. I guess it's down to personal choice.
I'd just like to add that whilst there are big changes in the way the big 3 shift, the two models in question are very similar. Red utilises double tap meaning no change in hand position for up or downshifts and Ui2 has buttons meaning again that no change of hand position is needed either.
SRAM has double rap on all of it's road groups and have a more mechanical feel in the levers leaving you in no doubt that you've just shifted but is such a way your legs don't notice it.
Shimano have dual control meaning you can either have lever and hood button shifting or (more commonly) use the brake lever and a small lever behind the brake for up and down shifts thus requiring a small but minute change of hand position. Shifting feel in the levers is noticeably smooth too which isn't for everyone..
Campy have the lever and hood button layout also but has the ability to dump multiple gears both up AND down the cassette whereas the former 2 only allow multiple shifts up the cassette. It also has very mechanical feel to the levers.
Regarding the 2 in question though my opinion remains firmly with Red however it's only an opinion. Contrasting to the above post, everyone I have known to ride with Red has stayed with Red. I have ridden Di2 and Ui2 and still prefer it so you're bound to have your own opinion too if you ride them both first but the only advantages you gain with Ui2 are electric gears and thus, the ability to program them how you want (if you know what you're doing) Red is considerably lighter, more aero and has a lot more trophies under it's belt as a result.
+1 to Psycling's comments.
It's not really about 'best', you should take some time over the decision. Weight difference is irrelevant unless you're racing at elite/pro level (and even then it's debatable).
Not answering the question, but just to say it's worth checking out Pauls Cycles, as they have some good deals on 2012 SuperSixes at the moment.
If you are mechanically minded i,d go for sram , but if not go for ultegra di2 once set up it goes on and on and on, if goes out of sinc takes seconds to put right