Cell wins with mid-diff at most. Whilst Trunks was surprised Vegeta was using Grade 2 straight away, I doubt SSJ Vegeta's advantage over initial 2nd form Cell was that great if Vegeta was still disappointed at Cell's power as a whole. Full Power definitely wins when taking into consideration not only was it a great enough boost Cell thought it'd be a game changer, but also has some good hax if the Taiyoken and Life Drain are used strategically.