Cannondale supersix vs Felt F5

I have been stealthily working towards an ultegra/carbon framed bike by upgrading my alloy Synapse and now am looking to move on to upgrading the frame. I know this wasn't a particularly logical way to do things but needs must and all that - cash flow is an issue!

Anyway I have been working on the assumption that a Supersix frame will be the way to go, probably second hand as a complete bike followed by transplanting and ebaying of surplus parts. Not least because it shares a BB30 bottom bracket with the Synapse.

But now I've seen the Felt F5 (also BB30) at a touch over a grand new and am wondering if that might be the way to go instead. Looking for something fast, light and not too harsh but intend running 25mm tyres so not overly worried on the last point. Although if I can't get clearance for the 25s I guess I might be!

Anyone have comparative views on the two frame sets? Alternative suggestions also welcomed but I do want to keep my SLK chainset.