OP...Ive had a Roubaix and now ride a Tarmac...

Like the posters above have said the Tarmac and Secteur have the same TT length, but the HT on the Tarmac is shorter. Also, in new Tarmac's the top cap above the headset is shorter so the steerer is going to be tight for any vertical adjustment. I have found this out in comparing my 2010 Tarmac to my 2008 Tarmac. I was able to get the same saddle to bar drop between my 2008 Roubaix and 2008 Tarmac, but would have issues doing the same on my 2010 Tarmac. The seat tube angles are stated to be the same, but I found them to be slightly different. This stems from the fact I was able to use a zero offset post on the Roubaix, but have to use a setback post on the Tarmac to achieve the same saddle fore-aft position.

Bear in mind, as well, that the Tarmac has shorter chain stays and I slightly steeper HT (I believe) which will give tighter handling. This will also give you a bit of a harsher ride that your body may not be ready for.

Myself I started with a Roubaix and rode that for a year and a bit before moving onto the Tarmac. At that point I was ready for a more aggressive ride and now do the same length ride rides on Tarmac as I did on the Roubaix.

My advice would be to tak the cash, and buy the Secteur...You felt best on that bike and already bought one bike that didn't' work for you...