cruz just won his primary a couple of days ago, so i don't see how he's going to help or hurt romney in november.
rubio, on the other hand, would likely be a net loss. first of all, yes, he's a latino...but he's cuban. and to the latino community there's a distinction. but that still would probably get romney a few more votes in florida, virginia and nevada where there are a lot of latino voters.
now for the downside...
he's got some really questionable financial dealings that haven't undergone serious scrutiny. yes, the media in florida was poking around last year....but that's not quite the same as full force of the national media turning over every rock.
his compelling personal history was a lie. it could be argued that without that lie he never would have been elected to the senate in the first place. maybe he would have, but we'll never know. what we do know is that he'll have some serious explaining to do about this.
he was baptised a mormon. how in god's name are the republicans going to have two baptised mormons on the same ticket. evagelicals might be willing to hold their nose and vote for one....but two? not likely.
he's green...and not in the environmental sense of the word. i really wonder if he's ready to handle the bright lights. he did an interview with politico a few months ago, and he was clearly upset by the questions and wasn't able to control himself.
so in the end, i think rubio would hurt more than help romney's chances.