As you told he made a great season, maybe you should keep him for 1 year or 2.
But, personally, I've never been a fan of Arshavin because he always disappointed me in FM 2009 & 2010, so I think you should sell him as you need to prepare the future. With the transfer fee, I recommend you to buy some wingers like Gervinho, Mata or Ganso if you use Arshavin as AMC.
In 4-5 season, Walcott turns into a big time player, so keep an eye on him as well ;D