i bought some Momentum for the very first time on Sunday (nearest Shell to where we were at at the time had run out of V-power). With clubcard fuel save points I was pleasantly surprised to see that it was only about £1.04 or so a litre, though to be honest i don't use the car enough to be stung that badly by V-power Nitro at £1.30-high a litre or whatever it is these days.
I was driving back through London so have no real clue if there's a difference in performance, doubt it would be anything more than marginal if there is notwithstanding the cleaner engine aspect you get with Shell over Tesco.