It's a tough one. I was there for a while. I was coming from a 300tdi Defender and posted a similar question. The answer I got back was the right one, it doesn't matter; both of them are going to blow your mind.
Money for me was an issue (a big one) and I was just stuck until an end of model runout sale came on and an evil enabler on AULRO (yes, you LR D4) offered to send me the list of available cars and prices he had because he had just ordered one. I found a 185kw model with everyting I wanted minus e-diff. Some words of encouragement from GGHaggis about being able to do without e-diff and I took the plunge.
Upon reflection I realised that I really wanted the 185kw engine and as I had not yet justified the extra expense to myself I wasn't signing up, but I obviously didn't really want the 2.7 in spite of all the common sense I'd read here. At the time not only was it more powerful but it was more economical than the base engine. That really disturbed me.
I'm so glad I went the hot rod engine. I can't tell you what a peasure it is to drive. In the country you just have to think "overtake this roadtrain/caran/campervan" and POW you are at 130 and moving back into your lane safe and sound.
You could definitely feel the difference between the 2.7 and the 3 right from the get go, smoother faster, better. Your decision is tougher because both options are the 3 litre engine and I reckon the difference has reduced, but for me I'd lean towards the rocket ship version. Thing is, I had some deal-breaker options. Fuiji White paint, Hi-Line sound system, and reversing camera. This car had all the must haves. I lost the e-diff but I could live with that. If it did not, then, no I would not have taken the plunge.
So if your demonstrator has all your non-optional options give it careful consideration. Of course the detuned engine is awesome and will be absolutely fine, but the high speed engine, is take my word for it, "whoah baby!" to a humble Defender driver like me.
