So what makes you take out the Venge v. The Tarmac?
i personally feel the venge is the better all-around bike -- for me, based on my riding and racing. i have the luxury to own both and got the '12 tarmac sl4 first (as an upgrade to an '11 sl3), but honestly had i replaced the sl3 with a venge i never would have gotten the sl4.
by no means am i saying the sl4 is bad -- quite the contrary! it is fantastic. however, i feel like the few things it does a little better than the venge (as noted above, low speed accelerations such as going from 5-8mph on a hill) are outweighed by what the venge does better (accelerations from 25-30mph). the venge isn't far behind at the low-speed, climbing stuff, but it feels quite a bit faster at the high-speed stuff. the aero benefit has, for me, been proven to be real based on riding the same courses many, many times, but even if it is a placebo i wouldn't care.
one reason i did not go for the venge first was that i was scared off by the "all aero bikes are too harsh (vertically) and not stiff enough (laterally)" rhetoric. i confess that this seemed logical based on tube shapes. i didn't have a chance to ride the venge before getting my sl4. early reports also spoke of aero bikes having a substantial weight penalty; i believe it at the time.
if it is harsher than the sl4, i do not notice it. it is plenty stiff for my riding and racing. the handling is every bit the equal of the sl4....in my hands, at least. as for weight, as per my earlier post it wasn't until i personally weighed everything and compared to the same manufacturer's top-level offering in the same size that the weight penalty proved to be pretty paltry. (i know, i know, 100-150g is huge in some ways, but many people made it out to seem like a venge could not be built up light.)
i would happily race either and had i not spent as much time on a venge i would not pine for it, but at this point that is the one bike in my garage that you would literally have to pry away from me.
while i have raced both, at this point i reach for the venge for racing, every time. i set up the tarmac as a training bike (a luxury, again i know), so now i generally have some easier gears on it and different wheels. what bike i reach for for training is more often the tarmac but it has more to do with weather and terrain choices.
hope that answers your question. i tried to give you some insight into my psyche.