Vegeta is much more ruthless. You can't point to that one example of Vegeta holding back since he only did to basically torture Cell in the first place. And he allowed Cell to become perfect out of pride. Don't forget Trunks did toy with both Frieza and Cold before killing them too. He even gave up his sword just to show his dominance.
Vegeta is much more ruthless tho. Killing Guldo when his back was turned, killing a KO'd Burter, sadistically killing Zarbon, Dodoria and Cui, killing multiple innocents just to goad a fight out of Goku. Trying to kill good Fat Buu after he saved everyone. Vegeta has Trunks beaten there
Everyone Vegeta face he gives them a chance, the same doesn't apply to Trunks, he kills them when he knows they are challenging. Same with the Androids, Trunks tried to kill them during the onset of the fight while Vegeta played around, even against the first set of Androids.
Vegeta only did those things to either A. Test his new strength, or B. Try to get a challenging fight.
Trunks has done similar things, and those things don't make a character any less ruthless.
Vegeta is the most ruthless in the series though. He killed Nappa, one of the last four remaining members of his SPECIES, just because Goku had made him useless by handicapping him. Not to mention he's killed people out of spite, sport, and even simply because he felt like it. I'm referring specifically to the instances of Jeice, Burter, Zarbon, Cui, etc.
Vegeta's literally blown up planets and destroyed entire species. He's not as ruthless as he used to be, not by a long shot. But he's still the most ruthless non-villain in the series, other than possibly Beerus.
"Why is everybody so ****ing stupid?"- Kim Jong Il, Team America.