What frame size?
It doesn't look like the 2015 Defy 5 has the PR3 tyres you cite in your other thread but the presumably cheaper SR-4I have a Defy 3 in m/l size, I'm 5'10 and find it very comfy, it's a fantastic bike, only thing is the oem rear tyre has started to split after about 400 miles, however I have some continental GP2000S 2's to fit on it this weekend, if you can get the size, go for it, you won't regret it.
Perhaps you mean 6ft5'12" and on a m/l
I'd certainly pay the extra, others wouldn't. It'll be easier to sell on once upgraditis inevitably sets in with a carbon fork IMOCarbon fork starts on the Defy 4, both are 16 speed claris its listed at £76 more than the 5.