It dependsTM .
Does the SB act first preflop, or the UTG player? If SB, then the button straddle gives an advantage to all of the other positions–there are two fewer players left to act behind them, so they can widen their opening ranges when the blinds fold. Also, the blinds have to play very tight in this configuration. If UTG starts, then the other players don’t gain an advantage.
Is the straddler aggressive with his straddle raises, and always defending when raised? We can limp/reraise with strong hands if so.
In any case, raise size should increase, because the straddler has position, unlike the regular blinds. In addition, with 3 blinds, the pot is bigger, so a pot-sized opening raise would be
$21 (edit: $18). I’ve noticed that some players still raise to $12, which seems very bad, because it gives the button very good odds to call in position with speculative hands. $20-25 opening size with a range adjusted for shorter stacks is good.
I usually only button straddle if the players to my left are good, aggressive players or maniacs, such that I want to force them to act first. I would probably never button straddle in the UTG first configuration.