Strålman is an extremely underrated player right now, but we'll see how that is at the end of the year when a new contract is due.
I'm not sure he is a better D-man than Girardi, but he is definitely a more versatile one. Girardi is a great complimentary D-man who has played excellently with Tyutin, Staal, and McDonagh. Give him a less reliable partner however and the result is a total trainwreck - see the Del Zotto-Girardi pairing.
Strålman is less dependent on his partner, and performed extremely well with every defenceman he played with in 2012-13.
I'm not sure if Strålman could handle Girardi's first pairing minutes as well as G - probably not. But in a salary cap world, Strålman at $2.5M is more valuable than Girardi at $5.5M.
However due to MDZ's inability to play the right side effectively, he is still my first choice of D-man to move.