I pretty much always go for the higher octane variety, usually Momentum as there is a Tesco half a mile down the road. I couldn't tell you if it makes any real difference as I have not used lower octane optiions in the car at all. Besides, this early in the car's life small improvements in power/mpg are likely down to any number of other factors.
Quite apart from performance, Tesco claims that the additives also help maintain a clean and healthy engine. Again little scientific evidence of this though.
Whether the cost is 'whole lot more' is fairly subjective, at 5p a litre over regular thats around £2.50 on a full tank. For the price of a coffee i'll take the risk that it's all hokum